There is a choice between heroics and flex called "LFR" (looking for raid) and it is definitely a way to box raids. Like LFD (which was introduced at the end of wrath) these groups are created automatically, you just queue up when you have the right gear level.

I'd say bring the ranged class you like the most. Dps output is closer this expansion than it has been in the past, and the #1 indicator of how much work you'll do in tweaking the dps rotations is how much you like the class.

At the moment (until 6.0) disc priests are the best boxing 5 man healer because they can do passive smart healing while DPSing. Shaman drop something on the ground, which is a mechanic that isn't fun for boxers so I wouldn't heal with them, and I don't know anyone that boxes with a paladin healer although I think I'm going to try it soon just because that is the only healing class I've never played.

Tank with what you want, with the possible exception of brewmaster (a lot of people tank with brewmasters terribly and don't realize it, so you could be making your healing situation a lot harder than necessary and be unaware).

Shaman/Mage/Hunter can all do blood lust now, it is still handy to have.