I have a 6 cylinder 3.0 liter Z3 and I had a 1985 325 also. The 325 I had had some good power, even thou my was an automatic it was just fine for me. Nothing like my Z3 but fine. If you were happy with your old 1985's power get the 4 cylinder standard and you will be fine and save some money. If your 1985 left you wanting more get the Z3 in a 6 cylinder.
2001 BMW Z3
3.0i straight 6 engine
M badge series
Dinan cold air intake, Dinan software, Dinan High Flow Throttle Body, Dinan stage 2 suspension upgrade, Dinan Strut Tower Brace, 18 inch wheels, and don't forget the fancy pedals.