Just for a little balance...
I've only had my 3.0 since beginning of March but I'm only 5' 6" so plenty of space in it for me, which is the major criticism from taller drivers. Mechanically the water pump impeller is made of plastic and can fail, so look for receipts confirming it has been replaced or bank on having it done (though the lower mileage on your example might help here, but my 60k example had been done just prior to purchase). Check for split pipework around the inlet side (especially smaller tubing underneath which is harder to see). This leads to rough running/poor idling. Camshaft sensors can fail leading to poor running/bad starting (inlet sensor) or just lack of low end torque (exhaust sensor).
Front wishbone rear bushes get soft (as much due to age as wear) which leads to sloppy handling/bump-steer/wheel vibration. Cheap enough to buy if you can do the work yourself, but not massive expense at your local indy. I fitted Powerflex ones to mine for a good improvement without spoiling the ride. I'd highly recommend adding a strut brace as I bought a used one and it firmed up the front of the car noticeably as well. [I've done all my jobs one by one to see how they effect the car, so I also noticed that the front drop links made no noticeable difference to my car for example]. Bodywork should be good as they are galvanized, so any rust might indicate a poor repair (though you will see some under the bonnet judging by the 8 or so cars I viewed when looking for mine).
Best place for information is the ZRoadster.net website with a Z3 'knowledgebase' which has more details than I've mentioned. Overall I think it's a great car, if you like the looks (which some don't to be fair) and can fit inside. I haven't had any issues from other drivers that I could claim were due to it being a BMW, but then I drive a 320d during the week and try to be an atypical BMW driver myself.
Finally, the 6 cylinder models sound lovely and all have a tendency to pop and bang slightly on the overrun.
I bought mine as a stopgap to getting a TVR Chimaera 450, but this will mean finishing and then selling my RS2000 to raise the money and to provide a garage for the TVR (the Z3 sits outside). I don't know if I'll be able to sell the RS2000 (it's been in my family for 32 years) but I think I could live with just having the Z3 and RS2000 instead.