Actually, the 323's and the 325's BOTH have the 2.5L inline 6er engines. The 323 is a very underrated car! I have an E36 323 and it's pretty quick (15.0 in the 1/4). Out of those cars I would say depending on what you want to do here are my choices:
1) You are NOT modding and just driving the thing - get a 328
2) You are planning on doing mods - get the 323 and have some cash to spend
Don't get a 320i.......no power what so ever, and the 325's are just newer they are basically the same as the 323 just cost more! Hope that helps a little bit. I am sure you'll get more help too!