wiki can and will be helpful at times.
As for last hitting, well, yes, runes do help, especially if you're an AD based champion such as Vayne or Jayce for example, but they aren't fundamental to last hit. Last hitting is something you need to learn for yourself, runes do help yes but the amount of help they give is, well, not as big as one might consider. You just need to train and you'll get better with time, no need to rush because you won't be able to last hit properly and efficiently in one week or maybe in a month.
Doing these things in Dominion or PG(Proving Grounds) is more profitable than doing them in SR I believe, simply because the games last less than 25 minutes, most of them do atleast, farming isn't something you need to worry about and it's basically a map that you require skills and need to know how to play champion X or Y to play well, SR does this too but I'd say it's faster in maps that last less time and can get you atleast 4 items in a fast way.
Yes, he's worth buying if you're into ap assassins.
As for team comps, champions; well, I don't really care, take whatever you want, even Nasus(... <-<), though the meta around Dominion is usually bruisers, as always been bruisers as far as I can remember, assassins, especially if you go bot, APs with high burst or good sustain(Akali, Kat, Kass, etc), high mobility(like Vayne's Q or Kat's Shunpo) and basically any champion that runs like a motherfucker(Shaco, Eve) but those that run fast or move fast, with no help of items, aren't very popular but still good choices, especially if you're good with them(Shaco and Eve for example).
As for builds, it's pretty much the same as SR but getting boot speed upgrades is usually something you should do after your second item or so, though 15+ MS isn't that fundamental, depends on what champion you have really; situational.
Dominion can sometimes be confusing because if you control a lot of CPs or just three or more, people can't be still and wait to win the game, they want fights, so some teams actually lose because they don't defend their CPs at all when the enemy is going to attack or is going roaming(Shaco is good for this and so is Eve).
Basically, get CPs and defend them, roam a little bit but not to much, keep the pressure in the entire map, you'll win. Remember that ganking bot is very beneficial, even if you don't get the CP, assuming no one dies in your team.
Sorry, there's no TL;DR for this kind of stuff. :0
You guys make me type so much!