Asides from the usual items to check as mentioned above, the one thing I will suggest to look at with great detail, is the roof actuator (motor). I saw this problem a fair few times during my time at BMW. Amazingly the E36 only has one actuator (under boot lining, left hand side), and these are prone to packing it in if the roof has been used lots. It's the plastic teeth inside the actuator that disintegrate, and the only fix is to replace the entire actuator (cannot be repaired). When the E46 convertible came around, the engineers wisely decided to install two actuators.
Another thing to look for is whether there's any bunching up of wires around the compartment for the roof/boot. This can lead to a few headaches if not sorted too. Last but not least, always test the roof in the wet weather...if it ain't raining, a high pressure garden hose also works. Look for vulnerable fitment in particular at the rear top corners of the front windows, and any stitching which looks suspect. Don't be afraid to haggle either coz of the weather and the fact that the E36 Convertable isn't as good a drive as the coupe or sedan (flexing body). Buy as much underbody and under bonnet bracing as you can.
Hope that helps!
