I'm planning on spending $15k, but I will probably do it not this year but next, or maybe even in the spring of 2007. You can probably find a 328 for 15k, but that is asking for a lot. You will not get much out of a 323 or a 325, so I say you make sure to set apart enough for one with a bigger engine, with a 330 being preferable. I would raise my budget a little bit and wait until a 2002 330 fell within reach.
If you care more about preformance, etc., as you said, then get an e36 M3.