look out for rust around battery tray and fuse box tray, floor pans etc etc.
as with any car, if it smokes then theres a problem.
all depends how much money you have to spend, in my opinion the more money you spend the better the car should be. am not saying that it will be!
if your spending alot of money on an rs then look for one with fairly low mileage, low number of owners previous mots to back mileage up, stamped service history, original paintwork, decent interior.
a good tip i learnt was, when you go to view a car always make sure its stone cold when you view, so you can see if it warms up well and starts well with no rattles or smoke etc. when warm try pulling the oil cap off it should cut the engine out, if it dont then it has been set up with an air leak and may be breathing heavy.
if the car wont start when its hot it could be the starter motor baking as it sits beneath the turbo.
how much do you ave to spend? if you have 2 grand you may get lucky and find a decent 1 but for that price expect to pay out on some work!