^^ in that case, I would go with the cleanest, lowest mileage sedan I could find. The 318i has very good fuel economy (Delmarco, a member on this forum, says he gets about 36 mpg on his), and -- being honest -- enough power to get around and rev up the engine every once in a while. However, you can get a 328i or a 325i for close to the same price. These latter two give you a decent amount of extra power and, more importantly, torque. The official numbers are 29/21 mpg hwy/city, I believe. I get about 22mpg combined on a 328i that gives me 29mpg on road trips. Another good thing about the 328i (model years 1996-98) and 325i (92-95), is that you are more likely to find one with the luxury features than if you are looking among 318i's. The 1992 325i models have a weak subframe, that tends to crack, so you should watch out for those and ensure that the frame has been properly reinforced, or prepare to have to pay to do so.
My personal recommendation would be the newest 328i you can find. Keep adding any specifics (e.g., your price range) so that we can help you narrow down your search.