I have just bought a 2007 2.0i Z4 and I suggest to check this things: for the exterior check the roof conditions (open/close, noise, gaskets and how the door window closes); check the battery compartment in the trunk (if the car had a significant incident in this area its shape won't be perfect...); in the engine bay look for fluids leaks. During the test drive try all the gears (also try to accelerate from low RPM and the reverse looking for strange noises). If you buy it, check the roof water drains.
About the 2.0: I know that is not a straight six, but, in my opinion, it is a good engine for this car and is really enjoyable (also compared to other four cylinder.engines).