ability-scoresdnd-5ehalf-orc
The title might be confusing, I’m not really sure how to word it…
I’m making a half orc character. It says that he gets ability score increases on Strength and Constitution.
Does that just mean that his ability score in Athletics (determined by Strength) goes up but not his main Strength stat?
Or does it mean that his Strength stat goes up, changing the ability score anyway?
As okeefe points out, undead in Pathfinder use Charisma to determine hit points, saves, and special abilities related to Constitution. However, that rule does not mention anything about replacing bonuses to Constitution with bonuses to Charisma. If you have a non-ability in Constitution, then you simply don't gain the effects of that bonus.
Also important to note, from the undead subtype:
Immunity to all mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms).
And from the Barbarian Rage entry:
While in rage, a barbarian gains a +4 morale bonus to her Strength and Constitution, as well as a +2 morale bonus on Will saves.
Note that undead are specifically immune to morale effects, and the Barbarian's Rage gives morale bonuses. In this case, regardless of whether or not the stat boost to Con shifts to Charisma, undead gain no benefits from raging. You may want to talk to your DM about houseruling some of this stuff if he wants your Barbarian friend to keep being effective.
Tl;dr: Bringing Intelligence up to 20 is necessary at some point, but doesn't have to be immediately. War Caster and Resilient are both great options and will help you maintain concentration. Heavy Armour Master and Lucky are also great options but won't have much impact on concentration. None of the other feats seem particularly worthwhile for you. Personally, I'd probably take War Caster and then decide between Heavy Armour Master and bringing my Int up to 20.
I'll address your points one by one, starting with:
Bringing Intelligence up to 20
You definitely want to bring your Int to max at some point, because it controls your primary means of attack. The question is whether you want to get it now, or leave it till later. With that in mind, consider what 2 more points of Int actually provide:
None of these are huge benefits in the short-term. Long-term, this makes you 5% more effective at most of the things you specialise in. (Warning: gross over-generalisation.)
War Caster
You don't intend to have both hands full, so this only gives you advantage on Con saving throws to maintain concentration, and spells for opportunity attacks. However, both of these are strong benefits. Advantage to maintain concentration needs no explanation, but spell opportunity attacks might not be as clear.
To start with, you're planning to be in the thick of the melee, so you should be getting opportunity attacks often enough to want to use them. Unfortunately, you aren't planning on using a weapon, and your signature spell Vampiric Touch won't work with opportunity attacks. (It specifically requires an action on your turn to make an attack with it.) So in your present state, you have no use for opportunity attacks. Giving yourself an option could pay off.
On the other hand, opportunity attacks aren't the only reaction, and you might want to save your reaction for spells like Shield or Counterspell. That said, these require resources, whereas with War Caster you can always use your opportunity attack to cast a cantrip.
Heavy Armour Master
As you say, the Strength is negligible where you're concerned, so the question is the damage reduction. This is a great option for someone planning to be in melee - 3 damage off every non-magical weapon attack adds up to be a lot of damage very quickly.
For concentration, it's a bit weird. It won't affect the vast majority of Con saving throws you make to maintain concentration, since the damage has to be at least 22 for it to have an impact. (Many high-damage attacks are magical, and it won't affect those either.) When it does affect the DC, it will only reduce it by 1-2. So as an option to help you maintain concentration it's a bit of a bust. It's still a great defensive option though.
Keen Mind
I can't say any of these features have ever come up in a game I've played. They're all highly situational and I can't find anything to redeem this feat other than "it's kind of cool, I guess".
Linguist
3 languages is pretty good, but there are spells that take care of this sort of thing for you. I've never seen a situation in game where a cipher would've been handy. If you're looking for a way to send messages to someone that no one can intercept, there are a variety of spells to achieve that.
Lucky
3 rerolls a day that you can choose to spend once you've seen the result? This is huge. You can spend them on an attacker's roll against you? That is just nuts. If you get critted, you can just wipe it away. Note that its benefits aren't nearly as potent on rolls that you make with advantage, but they're even more helpful on rolls you make with disadvantage. As far as concentration is concerned, you can fix a failed concentration roll 3 times a day, but you're likely to be making a lot of them, so something that isn't a scare resource and benefits all of them is probably worth getting.
Observant
This feat is frighteningly good. Just not for you. For a character who's maxing Wisdom and has proficiency in Perception, this makes them a portable radar station. (One of my players, the cleric, took Observant and has been making my life hell spotting every stealthed enemy and trap a mile away.) For you, it will make you almost as good as they were without it. If your party lacks a "sensor" character, it could be worth considering. Assuming you have one, I don't recommend this.
Everything in the above paragraph is based on Perception, without considering Investigation. That's because Perception is used far more often than Investigation, and passive Perception is used almost constantly whereas passive Investigation rarely sees the light of day. This can vary from campaign to campaign, obviously, but in general Perception is better.
Lip-reading is a cool little feature that I have seen used several times in intrigue situations, but I don't think it's worth spending a feat on.
Resilient
For maintaining concentration, this is pretty strong. More reliable than the advantage provided by War Caster, and scales better as you level. As well as that, though, there are a lot of effects that trigger Con saving throws. Most diseases, poisons, many spells, and a lot of monster effects. There's a saving throw for every stat, but Con, Dex, and Wis are still the most common, so this is definitely worth your consideration.
Ritual Caster
First and foremost, you have every benefit of Ritual Caster (as far as spells on the Wizard list are concerned.) Second, the benefits of this particular feat are highly dependent on what scrolls the DM feels like tossing your way. So the only quantifiable benefit of this spell for you is learning 2 1st-level spells from another class's spell list. Unless you can see something you really want (I can't), I can't recommend this.
Besides that, most rituals are either available to the Wizard or can be effectively duplicated by the Wizard. The only one I can see that you can't produce for yourself (barring a Wish, obviously), is Silence, which I can't quite see how you would use as a ritual. (It's more of a combat spell, and the 10-minute casting time is inconvenient in combat.)
Best Answer
You only have six ability scores -- you have modifiers for each ability score (like strength) and modifiers for each skill (like athletics). When you create a character, you'll choose initial values for each of your six abilities (either by rolling dice, or using point buy, or using the standard array). The race you choose will modify some or all of those values, and as you level up you'll be eventually able to increase those values.
Your score in any one particular ability determines your modifier for that ability (subtract ten, divide by two, round down).
Your modifier for skills are equal to your modifier for the relevant ability, plus your proficiency bonus if you're proficient in it.
So for example, let's say your starting strength was 15 at character creation, plus 2 from half orc. So at level 1 you'd write "17" as your strength and "+3" as your strength modifier. Your proficiency bonus at level 1 is always "+2", so your athletics modifier is either "+3" or "+5", depending on whether you chose athletics as one of your initial skill proficiencies.
Your skill modifiers are constantly "updated" to match the relevant ability modifier, so if later on your strength goes from 17 to 18, those +3 modifiers all become +4.