Well it really depends on what you want from the car. If all you want to do is look cool and don't mind a lack of acceleration than get the 318 or 323, but if you enjoy the kick in the pants feeling of straight six torque then get a 325 or 328. All the models have problems, and if you really want to look into it get the book BMW Buyers Guide. You can find it on Amazon.com. In my opinion the 325 is superior because it uses timing chains instead of belts, plus it has more power. You might even want to go with the more up to date 328 with the OBD II computer system.
If you have any more questions, feel free to send a private message or email me.