Stihl 63PM chain (without the crappy triple hump safety straps that make it cut like a Home Depot Poulan or something). The 1/4" chain Al suggested is a horrible suggestion. It's not readily available, nor is it strong enough for the power of the 200T. 1/4" chain hasn't been available except for special order for about 20 years now.
On the carb, some people have good luck rebuilding them. I'm not one of those people. I've probably installed 30 carb kits in my life and gotten satisfactory results about 4-5 times. On the 200T particularly, it seems to develop an air leak around the butterfly shaft or something to where it runs lean and you can't richen it up enough to run right.
Carbs are about $75 and it takes 5 minutes to swap it out and dial it in. The alternative is buying the kit, spending an hour disassembling the carb and trying to get everything back together perfect, only to have about an 80% chance that it will still run like crap. A dealer will charge a half hour to an hour labor plus the kit, which puts your cost higher than just buying a carb. Therefore I am inclined to just buy the carb and save the old one for a future project. Maybe one day somebody will come into my shop that is better rebuilding carbs than I am.