318's were made from 84-85, then they came back mid year in 1990 as 91's with the M42 engine (also used in the e36's till 95). 91, i think, was the only year for 318i/is', but the cabrios lasted till late 92 or even 93.
as far as looks goes, the front valance changed in 88, but the bumpers didnt change till 89.
both the 325 and 318 engines are very strong and probably wont cost you more than regular maintenance to keep running well. just look for everything you would on any other car and you'll be fine in making a decision