i'm a long time vw owner, i know these cars pretty well.
the things to look out for on a 2L 16v 91 Passat would be..
check the timing belt, if it snaps on you, it will bend the valves in your engine, also they hate any oil filter that doesn't have the valve, watch for lifter tick, it's pretty common, knock sensors go bad sometimes, those passats have a differenty style transmission than the other VW's, so you can only use cable shift transmissions. the 16v, also has 10.1 brakes, which means, you can only fit 14" wheels on it at the smallest.
they are good cars tho, i've owned 5 of the 85-92 Body Style Jetta's, 1 95 Jetta, and a pair of Scirocco's.
prevenative maintnence is key with these vehicles.