Rust -
Sills, Sunroof (just behind, but also the drains inside the roof), battery tray, fuse box then all the other usual places.

I always think with a S2, get the best shell you can afford, engine imo is easier and cheaper to fix/do up than battling with tin worm.

I think the standard management is good up to about 180, although I know folk running a 5th injector on ke-jet with ~250bhp so its not gospel.

Same as any other car really, if they are modded by monkeys then its gonna give problems....get a reasonably tuned one with money in the right places, and check little things like bushes, are they upgraded etc to make sure its got good bits underneath as well.