I've had an '01 Z-3 3.0i, for a little over 8.5 years, if you need to carry anything other than 5 bags of groceries, you're out of luck .
I've only got 7600 miles on it, so I can't tell you about any problems that may arise in a higher mileage vehicle, the independent BMW shop that I take it to, when it does need attention, the owner says that there aren't a lot of problems until you get around 100,000 miles .
Consumer Reports long term view of the vehicle, puts it at the top of the list for value, and reliability .
There seems to be a bit of problems with the interior, but I can't remember specifics right now .
Stay away from the 1.9 liter 4 cylinder vehicles, as they have a hard time getting out of their own way .
If you live in an area where salt is put on the roads during the winter, I would have a shop that is knowledgeable about the car, and do a good inspection on it, as there were problems with body integrity due to rusting, in the rear shock, rear axle area, didn't get real concerned about that, due to living in a desert, and the car has never been in rain, let alone snow .
The car is a blast to drive, and in wet conditions, I think you may get into trouble with too much throttle, even with the Dynamic Stability Control on, at least with the 3 liter engine .
I have the 5 speed manual, so I can't comment on the Tiptronic, or Steptronic automatic transmissions .
I don't know what years, or engines this was applicable to, but on the 3 liter car, the front and rear tires are different sizes, so you can't rotate the tires .
The throttle is a 'fly by wire' type, the gas pedal is connected to an electronic transducer, and that feeds an electronic signal to the engine management computer, so a 'lazy' throttle can be noticed when you first start driving it, but you seem not to notice it after a while .