Price is low - car looks like it's in pretty good shape. Most of the stuff you'd look for is the same as any other car. Check all the fluids for contamination, check undercarriage for rust, verify VIN on all panels,
run VIN through NICB to see if it's ever been totalled or stolen. He's been remarkably up-front about condition too, and fixed a lot of the little problems these cars have. He seems to really know what he's talking about. I'd do a quick Google search on all the usual E30 sites to see if the owner has posted anything there. For instance, I found
this at R3v, likely not the right car, but it could be.
Here's a BMW VIN decoder that will give you the original build sheet (including originally-installed options!):
http://www.bmwarchiv.de/vin/bmw-vin-decoder.htmlAs far as E30 stuff, check the front suspension bushes and joints for wear. Look for any rust damage in front of the doors, at the base of the a-pillar. Lower door frames & rocker panels are vulnerable to jacks & lifts. Rear body near subframe bushings can rot out, fuel tank commonly gets dented. Happily the E30 is a stout little car, but always take a flashlight and a magnet.
As for M42 stuff, overheating is a big problem. Check coolant and oil for cross-contamination. Any milkiness under the oil fill cap is cause for concern. Listen for cam noise - ask if the M44 tensioner has been updated. Ask him if he's tightened the upper oil pan bolts - they tend to work loose and can starve the motor of oil. Hopefully all it needs is a new clutch.
Good luck!