As a Holy Paladin, look for gear that has Intellect (increases Mana pool), spellpower (bigger heals), crit and haste. don't specificially gear for MP5/spirit. Unforunately for levels 60-70, gear will be a hodge-podge. Look at cloth, leather and mail gear as well, as some of the pieces are better itemized for healing, then the plate items available from outland. Keep you divine plea up, and use judgement of light up to help your healing and keep your haste buff up. Switch to judgement of wisdom if you're mana is getting real low. Also decide if you want to spam Holy Light (more crit) or Flash of Light (more SP). Check out EltistJerks.com, their fourms have a lot of useful info on gearing, though it is aimed more at end-game raiders.
This question gets a bit tricky to answer... Starting from scratch, if you buy just vanilla WoW, you can experience the original world as it has been destroyed by the Cataclysm expansion. You can level up to level 60, but cannot access professions, areas, races, or other such things from any of the other expansions.
In order to access Outland, jewelcrafting profession, and Draenei? / Blood Elf races and starting areas, and level 61-70, you will need vanilla WoW + the Burning Crusade expansion.
In order to access Northrend, Inscription profession, and Death Knight class, and level 71-80, you will need vanilla Wow, Burning Crusade expansion, and Wrath of the Lich King expansion. Oh, and a prior level 55+ character on your account to play as a Death Knight.
In order to access the misc Cataclysm level 80+ areas, Archeology secondary profession, Worgen / Goblin races, and level 81-85, you will need vanilla WoW, Burning Crusade expanion, Wrath of the Lich King expansion, and the Cataclysm expansion.
Those are the restrictions. You can play on whatever realm you want with whoever you wish. You just may not be able to reach all the areas they can.
You can trial every bit of it with the 10 day trial, though I don't quite know how they treat it when your time is up.
Best Answer
1. Own some non-twinks. This is the main reason, since it's easy to set up the character and PvP alts can be played on and off. It's far easier to get into the upper echelons of power in a level bracket that is not the top one, you just have to stop leveling and get gear levelers would usually skip. Twinking is an easy ticket to dominating low and mid level battlegrounds, with your only equals being other twinks.
2. Avoid the arms race. Getting to max level means farming Best in Slot equipment for months, enjoying it for a while and then having it become outdated as the new patch rolls out. Stopping leveling at, say, 70, allows one to play with the content of that particular part of the game without having the rug constantly pulled under them.
3. Experience old content. For example, you want to enjoy Ulduar. As a level 85, raiding the place would be a very different experience compared to those who would try it at the usual level of 80. You could try doing it at level 80 without twinking, and you'd either use quest greens from WotLK (too weak) or from Cataclysm (too strong), not to mention you'd probably level up midway. If you're a purist, you'd want to freeze at 80 and do it with the gear players used back when the raid came out. There are entire communities dedicated to playing outdated expansions with only gear from those expansions, for both reasons 2 and 3.