Are the multiplayer servers good?
Sadly not, they're awful. At launch it took many people almost hours to find a single match, and although it has gotten better with patches to fix the broken matchmaking, given that it's almost two years old now, the population will be smaller meaning that it'll be quite difficult (although not impossible - I've played it online a few times recently) to find matches.
That said, lag within matches is impossible to predict, as I'm fairly sure that the multiplayer is peer-to-peer, meaning that one player in your game will be designated as the host, and the whole game will essentially "run" off of their Xbox. Therefore, if they've got a bad internet connection, the game will be laggy. If not, it'll be fine.
Are the Sangheili playable?
Yes. In the multiplayer of Halo 2, Halo 3 and Halo 2: Anniversary, you can select to play as an Elite rather than a SPARTAN. However (and this applies to all of the games in the collection) you no longer get to pick individual pieces of armour, as you did in Halo 3 for Elites. Instead, you pick one full body set, and each armour piece will be the one from that set (or, in some cases, it will use the default armour piece).
Does my saved data transfer over, or must I start over?
Your saved data does not transfer, however it hardly matters, seeing as everything in the game is unlocked from the very start.