What's a 911 worth? How long is a piece of string?
Values vary greatly, especially in the older car market. You can pay $25K for ANY 911, regardless of age. Sometimes a 79 911 Turbo will sell for more than an 88. It's a weird market, and there are only a couple hard and fast rules...
1. Modified cars usually sell for much less than original cars, unless the mods are tuner mods that are "above reproach" meaning RUF or factory specials. EXCEPT...
2. Cars with mechanical mods like updated tensioners and other such stuff are desirable.
Modified bodywork is typically not worth anything and owners usually waste their money unless they find a poseur kid that is dying to pretend he's got a turbo and only wishes to pay 1/2. UNLESS...
I know an engineer with an SC model that has turbo bodywork 'cause he needs it. It's got 934 brakes and other bits and a 450hp Andial engine with RUF six cogger. The car has $70K+ in mods and would probably fetch $40K.
That 79 might be worth the money if everything is done right and the car is pristine. But, in the right market, a bone stock well worn 79 SC can fetch close to $20K, or sink to $15K or less. Look up a 79 911SC in the "blackbook" and you'll see what I mean...
yes, I know about the book, don't write, I was being sarcastic...