Repair or Replace! This is a major question.
Recently, I had my 240D's body fully redone. I took it to many shops and was told something different at each one. Cost became the number one factor.
Most shops were quoting prices to repair the rusty fenders, but I looked around the junk-yards and found replacements (myself) in near mint condition for $100 each. The $400 I spent for the parts shaved about half the cost of the body work - including parts! Total cost $1900 and the lowest body shop quote for repair and paint was $3750.
Bottom line, shop around... Many times you can get used body parts, in great shape, at less cost than a quality repair.
Just my $0.02 worth
76 240D - 550K miles
78 300D - 200K+ miles