From my experience, one of the most useful tools for an artificer is a Dedicated Wright (ECS p. 285). Get one of these and carry it around in a cart, or later a portable hole or something, and it can build stuff for you as you adventure. The Exceptional Artisan feat cuts the time by 25%, but personally, I'm not a fan of it, as the Wright covers most of the issues here.
For divinations and knowledge... Well, you're an Artificer. At level 6, you get Craft Wand as a bonus feat, letting you begin to make eternal wands, which are amazing for this. An eternal wand of Augury costs 5,695gp, if I did the math right, so you can make it for 2,847 gp and 5 sp in a little under six days. After that, you've got a 2/day "is this a good idea?" stick. Note that since you don't actually cast arcane spells, you will need to roll Use Magic Device each time you want to use one. However, because these are mostly out of combat things, you should probably be fine unless you roll a 1. If you want to take 10 on that, there's a feat in City of Stormreach (Hardened Criminal, iirc) that lets you pick a skill to always be able to take 10 on. Requires Iron Will.
The Oracle domain gets Divination as a 3rd-level spell, so you can make one of those as well (base price 12,150, so you're spending 6,075gp on it). You now have about the same amount of divinations as an equal-leveled Cleric or Wizard who's spending slots on it.
For knowledge, make an eternal wand or two of Guidance of the Avatar for 2,210gp each. Put cross-class ranks into the important knowledges and use your presumably-good Int mod and the +20 bonus to learn about the enemies you've divined with the other wands.
For detecting and preparing for an encounter, note that a lot of things are useful to have in general, even if you're not sure if you'll run into stuff. Scrolls of general-purpose answers like Banishment or Dismissal are good if you think you might end up fighting outsiders (and once you reach the level you could make them, that's a good bet for the near future outside of themed campaigns). Summons are also very useful. Make scrolls of Lesser Planar Ally for 600gp each, and then pay another 100-600gp or so to whatever you call for some sort of useful SLAs. Summon Nature's Ally IV can nab you a unicorn for the same price as a scroll of, say, Neutralize Poison, and it can also heal and temporarily turn off mind control effects. There are a lot of really good, open-ended spells to make scrolls or wands (or eternal wands) out of. You have access to all of the spell lists, and you can make stuff very fast with your Dedicated Wright.
The biggest issue I see is that, as you said, at later levels, the DM might not give you downtime. Having a Dedicated Wright is all fine and dandy, but it still takes time to make stuff. One solution to this might not fly in every game, but if you're trying to build a tier 1 artificer, you're probably already in a high-op campaign. Find (or make, once you're level 15 and can craft a scroll of Genesis) yourself a fast-time demiplane, and do your crafting there. Alternatively, if you're in Eberron, plane shift to Dol Quor and set up a base of operations there. It's got a 10:1 time ratio, and unlike Dream from the Manual of the Planes, it doesn't break anything you make there when you leave.
If you're not able or willing to plane-hop for more time, there's always an even cheesier method: self-resetting trap of Time Stop. Costs 76,500gp and 6,120 exp to make, and triggering it will give you as many rounds of effective time as you like, because Time Stop doesn't actually stop time, it just speeds you up. Nested Time Stops would just stack each one inside the first round of the previous, allowing you to get lots of time to craft at super-speed, then rest up and be ready for whatever you're doing. Heck, you could even go get new raw materials by robbing a bank or wizards' tower or something while sped up. This method is exceptionally silly, though, and really falls into the realm of Theoretical Optimization, so I suggest against it.
Also, it occurs to me that I didn't address your first question. My suggestion for that would be to first figure out what sort of artificer you're going to be. You might be someone who does lots of buffing through items, in which case, the Metamagic Item infusion combined with a wand/scroll/staff is really good. Metamagic Item for Chain Spell and Persist Spell on a buff spell like Polymorph, anyone? Suddenly everyone is a hydra, or something. If you're being a blasty artificer with Metamagic Spell Trigger, then your feats would be focused on that, etc. I have not actually played an artificer in a game, so I'm a bit unqualified to give advice on the actual build stuff.
For combating single opponents, one of the more potentially lethal archetypes for a magus is the Hexcrafter.
Hexcrafter to gain the slumber hex, and possibly the evil eye/misfortune hexes.
Take the Elemental Spell metamagic and the Maximized/Empowered magus arcana.
It would allow for a coup-de-grace requiring a pair of natural 20s to survive while still inflicting a great deal of damage.
The damage(done with a basic scimitar/rapier) with a strength score of 14, will be 2D6+4(weapon) + 15d6(spell). This will result in 11(average weapon damage) + 90(Maximized spell damage), for a total of 101. The DC of a Fort save vs Coup-de-grace is equal to the damage dealt(101) + 10, for an average DC of 111.
Best Answer
Bottom line up front: a Chaotic Evil belgoi werebattletitan could take ur-priest levels without any other class levels, and then have lycanthropy cured and the belgoi race changed to another without RHD, to end up as a pure ur-priest, at least through 10th level.
Ur-priest only has 10 levels to it, and even if you make an epic extension of it—as is recommended for 10-level prestige classes—you can’t take the 11th level until you are epic.1 Even a dragonwrought kobold can’t get out of that one, since Draconomicon only waives the 21st-level requirement from epic feats,2 not from other epic options including epic prestige classes. So if nothing else, you cannot be a pure ur-priest with more than 10 class levels.
Another sticking point here is the actual definition of prestige classes as “a new form of multiclassing,” that use “the rules for level advancement (see page 58 of the Player’s Handbook).”3 The very word “multiclass” indicates “multiple classes,” and if your first level were to be ur-priest, you would not have multiclassed into it. The referenced rules from page 58 are specifically about gaining levels after 1st, as well.
Moreover, officially, the rules require you to select your race and first class level at the same time, before doing anything else.4 So even if you could go directly into ur-priest, you would have to qualify on the strengths of race alone—a rather tall order when the requirements include feats (which you haven’t gained yet) and skill ranks (which you haven’t gotten points for yet).
The only conceivable approach, then, is as you suggest: by losing HD after you have gained ur-priest levels, removing anything you had before taking any ur-priest levels and leaving you with only those levels. Conveniently, ur-priest doesn’t require anything beyond raw hit dice to qualify: alignment, base save bonuses, skill ranks, and two feats that both have no prerequisites.
However, as you note, you are not allowed to start with no hit dice—you have to be something before you can contract lycanthropy. Whatever that this was will require at least one hit die, and that hit die won’t go away when lycanthropy does. While a 1-HD race does have that HD replaced by a class level, that happens immediately upon character creation, and can neither be delayed nor opted out of:
There is some hope though, from the very next line: “If a monster has 2 or more Hit Dice, it can start with no class levels (though it can gain them later),” emphasis mine.5 The problem is, unlike lycanthropy, other forms of racial hit dice can’t be easily removed.
Losing levels via energy drain is notoriously underspecified, and nothing explicitly says you can’t pick and choose which you lose—but even by theoretical optimization standards, that’s patently absurd. The rules include “The victim’s experience point total is immediately set to the midpoint of the previous level,” which strongly implies that the level lost is the most recent one.6
The one hope I thought of, though it is massively within the DM’s adjudication and is hard to call RAW, is reincarnate. Reincarnate specifies that “For a humanoid creature, the new incarnation is determined using the following table. For nonhumanoid creatures, a similar table of creatures of the same type should be created.”7 That means that—arguably—you could use a creature with multiple racial hit dice, qualify for and take levels of ur-priest, and then reincarnate into a creature has just 1 racial hit die. Then—arguably—that hit die is replaced by a class level, which—arguably—could be a level of ur-priest, since you already have levels of it.
If you can find a humanoid-type creature with lots of RHD then you are on pretty solid RAW footing to use the listed table, which include 1-HD creatures. I asked whether any such creatures exist, and it turns out several do, including the 4-HD belgoi.8 A belgoi lycanthrope could have quite a lot of RHD, and thereby qualify for ur-priest without taking any other class levels.9 All of those RHD could then be removed (by curing lycanthropy and being subject to reincarnate).
Even if you manage that, though, the rules around the replacement of the RHD from 1-HD creatures simply don’t cover this situation at all, which means we can’t really say that, RAW, it’s going to work in your favor—it’s going to need a ruling. Most DMs, unsurprisingly, are likely to rule that your 1 HD from your new race has to be replaced with a base class, and you can’t pick ur-priest for it.
But short of that, I didn’t think you can do any better. [It turns out I was wrong, as topquark’s excellent answer indicates—the ritual of vitality avoids reincarnate’s randomness and furthermore has an explicit example of RHD being removed and only existing class levels remaining.10
Even that, though, is still going to limit you to being a 10th-level ur-priest.
Epic Level Handbook pages 5-6
Draconomicon page 66
Dungeon Master’s Guide page 176
Player’s Handbook page 6
Dungeon Master’s Guide page 172
Dungeon Master’s Guide page 296
Player’s Handbook page 270
Dungeon vol. 111 page 88
A belgoi lycanthrope has humanoid and animal RHD, which means none of them provide a good Will saving throw progression. That means you need a minimum of 9 RHD to meet the base Will save requirement of +3. Taking the Education feat1 or Well Read feat2 gets you all Knowledge skills in-class, and the Flexible Mind feat3 can get you any two skills—pick Bluff and Spellcraft. The other two feats available at 9 HD would go towards Iron Will and Spell Focus (evil) as required by ur-priest. In total ur-priest will require 32 skill points, which means at 9 HD with 2 + Int skill points you need to have Int 14 to get them all—which is kind of a problem since belgoi apply a −4 penalty. Having more HD, though, reduces how much Intelligence you need, so you could freely go with a larger animal and need much less Intelligence. To get the minimum 9 HD, you could be a werelion, but if you want to just maximize your HD, you could be a werebattletitan,4 with a whopping 36 animal HD—then no matter how low your Intelligence is, the minimum 1 skill point per level is enough to get the ranks you need.
Eberron Campaign Setting page 52, Forgotten Realms Campaign Setting page 34, Ghostwalk page 31, Player’s Guide to Faerûn page 38. Note that belgoi are Dark Sun creatures, and that is conspicuously not one of the campaign settings listed here.
Dragon vol. 315 page 54
Dragon vol. 326 page 80
Monster Manual III page 38
Savage Species pages 149-151