The first thing is probably to get a decent baseline of where you actually are. I'd start of with getting the codes read in case there are any logged issues, such as a cam sensor. The Z3 doesn't always put the engine management light on if there is an issue.
I would probably also give the engine induction hosing a really good look over as the rubber is prone to breaking down and splitting, causing vacuum leaks. A full service (oil, plugs and filters) along with resetting the service light would be good too (the ECU retards the cars performance the closer it gets to needing the next service).
Next up would be a dyno run, the 2.8 is supposedly just north of 190bhp. If you get close with the dyno then you haven't lost any horses at least.
Only then should you start with the modifications. Usually these are as simple as improving the air intake, a remap and a free flowing exhaust - not big power gains but should make the engine a bit more driveable. Beyond that an intake manifold is possible for around and extra 30bhp.