It depends on how you drive the car. In normal city driving, the 325 will probably be plenty. If you're going to drive the car on the freeway and you need the strongest acceleration, the 328-330 would be a better choice. For most people, the 325 will have plenty of power. These cars are engineered to be superior performance machines. They are not one dimensional cars (some cars just go fast in a straight line, but can't turn or brakes very well). I have owned a 325i and I thoroughly enjoyed the car. I now have a 540i, which is one of the best driving cars I have owned. BMW cars give you strong acceleration, quick and precise steering, controlled braking, great suspension feel and comfortable, yet supportive seats.
My recommendation is to make sure you get the newest, lowest mileage 3 series you can. Also, make sure the car has never been in an accident - if you're not sure, take it to a body shop and have then run a paint meter test.
Good luck and I wish you many years of enjoyment from your BMW 3 series.
Greg