The '91 318iS is a very reliable car. It was only made one year for the US. Again, BMW engineers put their talents to the test.
It was intended as an entry level compact with some kick. The kick comes from the high reving 1.8 liter 4 cylinder engine. A 16 valve, (sodium filled exhaust valves) hollow twin cam engine that loves the higher RPM range. It's not much off the line but will hold it's own on the highway because of the power to weight ratio. This coupled with the 5 speed makes it a blast to drive. It's quick and nimble and well balanced for a small car. The "iS" model was the one that sported Recarro style sport seats, front air dam, rear heck spoiler, brake cooling vents, and shadowline trim, all from the factory. The suspension on this model was of the M-Technic variety and would provide great handling with a slightly lowered stance. Fuel mileage will average 25-28 mpg and a consistant 30mpg on the highway.
There isn't a lot of frills on the 318, but that also goes a long way in the reliability column. Some things to watch for trouble are fuel pumps, window switches, brake calipers, profile gaskets (that's a big one) and oil leaks from the front right side of the engine. Keep in mind that these cars are approaching 17 years old and they will require some work sooner or later. But the grin factor will sure to outweigh the upkeep on one of these models.
Good luck with your search for the right one!
JR