I'd at least pull the plugs and check them out. This should give you an idea of the combustion chamber prior to pulling the head. Also, do a compression check when the plugs are out. You never know, the owner may never have done a tune up and it may just be a bad plug, plug wire, cap, rotor, injector, etc. Do note that some owners only drive and never do any maintenance. So don't lead on any findings you get when talking to him.
But heck, get it for the $100, and if anything, you could part it out for 3 times that at least. Door idea: take the doors and make them 1/3 doors for that doors off feel with actual doors on.
I'd also do the normal review of the brakes, lights, wipers, ac, etc. Pull up some rug to check for rust on the floor. Take for a test ride around the block, and get a good feel about what the $100 is buying....