I still have a lot of questions on the details of what the mech was supposed to have ... can't figure out what was exactly done.
Setting that aside, it is time to fix it, drive it, and prosecute the bad guy if you can be reasonably assured you (a) have a case, and (b) can win the case. Spending more money (and, if you think $750 is a lot ... BTW, this sounded pretty reasonable for the items you broadly discussed, but again, I can't figure out what was actually worked on and the result) on an attorney to take a small time twit down (I also don't think I understand exactly what might have happened here ... newpapers are full of stories of people buying off the corner and finding out cars are from all over with hail, flood damage and repaints, etc) ... you will spend more than it is worth and in the end still be jacked that you got screwed ... except an attorney will have shared in your pain.
Fix it, learn from it, and persistently address whatever you can to ensure you pass on the experience AND the people and businesses that were involved (make sure you actually know what you are talking about and how to legally do this without getting in trouble yourself).
At no point, btw, have you mentioned doing any work yourself ... while this is obviously not something everyone does, driving a complex foreign car that is 16 years old without being able to understand and work on some portions of it may be unadvisable.
Good luck ... this time next year you will be very, very knowledgeable ... keep reading the many posts here ... I continue to be awed by the depth of knowledge and caring attitudes by everyone; posters, pseudo-mechanics, mechanic moderators, etc... and even the best parts guy - Phil!! - in the business ...
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg