My 3.0i has a different engine, so my experience on that is not relevant. A known weakness on BMWs is the electric window lifts, so make sure that you try yours. My driverside lift is failing on my 2001 model.
Before I bought my car some years ago, I test-drove a '99 2.8 liter with manual tranny. If everything is right, the car should be absolutely smooth up to 100 mph+ If there's some place you can do that safely and without fear of getting popped by the police, then I would do so. It will tell you lots about the driveline, wheels and suspension.
You didn't say whether the car you are considering is an automatic or a manual. If it's a manual, look for smooth clutch engagement, with no drama or shuddering. An older, low mileage car could very well have been driven in stop and go traffic, which creates the potential for lots of clutch wear, especially at the hands of a clumsy driver. My '01 has 56K miles, and a lot of it is that kind of driving. AFAICT, the clutch is still fine.