Go for a 1998-2000 M Roadster if you have the spare money lying around. It has the E36 M3 engine in it and they are very reliable. The later models (2001 & 2002) have the S54 engine which is the E46 M3 engine and the cost for parts are high. If you want just a regular Z3, go with the 2.8L or 3.0L. I know someone who has a 328 with the 2.8L engine and it is a hoot to drive. Just change the cooling system out and you are all set. I'm eventually planning on buying a Z3 coupe in the future. Here are three links that should be useful.
Used Car Classic: BMW Z3 Roadster & Coupe - Article - RoadandTrack.com
BMW M Registry - FAQ E36/7 M roadster
EDIT: I forgot to include this one.
http://forums.bimmerforums.com/forum/showthread.php?t=738323 <- Some people on that forum complain about everything though.