Chad,
As I mentioned in my email to you, it's better to pay a bit more for a good car, than less for one that needs work. And as CDN OpelNut has pointed out, the $3k CAD is somewhat less than the American folks on this site might otherwise pay. But it has been my experience that collectible cars in Canada usually sell for the same dollar "number" in $CAD as $USD. What's a fair price? Whatever the "market" determines. But the market for Opel GT's is pretty unstable, since there are so few cars for sale, and fewer knowledgeable buyers. Which is exaggerated in Canada. And as far as being "collectible", they definitely are that. So much so, that it can be hard to give them away, and pretty soon they "collect" in your front yard!
I agree with Gene. If it's a decent runner with a good body and fair mechanicals, it might fetch $3k, or as much as $5k in better than decent shape. But as soon as you factor in some body damage, the price drops exponentially. Even a minor fender bender costs $3000 to repair properly, if you were to pay someone. But, to be the devil's advocate, if you can do it yourself for a few hundred in paint and supplies, and buy some decent marker lights for a hundred or so, AND if the car is truly rust-free and a one owner car, it might be a decent deal.
You mention the engine was "rebuilt". That might be the deciding factor. If it was done properly, by someone who knows at least a bit about Opel CIH engines, that's worth another grand. Or at least it can COST that much, so it's "worth" half that. But if was just "freshened", with new rings and gaskets and not much else, it might actually be a negative. A badly re-built engine can cost more (MUCH more!) to "set right" than to do it right the first time. Can the seller provide details, and bills, of the rebuild?
JM2CW
Good luck!