The '98 model was the first year for the M version of the Z3 and has the S52 engine with 240 bhp and about 238 ft lbs of torque. Combine this power with a tuned suspension and a mere 3000 lbs of empty weight, and you have the definition of 'pocket rocket.' While the later 2001/2002 model year versions of the (Z3) M Roadster had a more powerful (and more complicated to maintain) engine rated at 315 bhp, the additional HP is pretty much useless on American roads and highways, unless you just liked having your neck snapped back from standing starts, that is. The M Roadster has a lower and more aggressive stance than the Z3, and it's a head turner. The M Roadster just plain ROCKS.
The power. The appearance. The handling. These cars are a deal now on the used market and will surely gain value as time passes. The FUN of driving the thing!
The Z3 M Roadster is no longer in production, but it could have used a padded center armrest and longer gearing. Neither one of these should be a deal breaker to owning one of these, though. BMW service/maintenance costs are notoriously high. Find a good independent repair shop for service work.