How to spot bondo
Take along a magnet. Magnets stick to painted steel, not painted bondo.
Of course, the fact that a part has been bondo'd does not mean that the car should be rejected, just than you should pay less for it, or avoid it completely if the bondo is extensive and the paintjob is recent.
The rest of your suggestions are also most helpful.
The best thing to remember is that the desire to buy a new(er) car is sort of like a disease, which often can prevent us from being as snoopy as we know we should be.
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"