Consider this, I spent aproximately 1000 hours doing rust repairs and paint on my SX/4 and while it's pretty good, its not as good as showroom mint - what's my time worth, at least $10 or more likely $20 per hour? What I did was the equivalent of paying $20,000 for a nice solid body. I don't consider an engine a very big deal, the vast majority of the labour and cost in a restoration is in body work if there is a substantial amount needed. I'd say a showroom mint SX/4 with complete good interior is worth at least $2000, even if it needs a motor. If I had the chance to do it over again and the opportunity to buy a showroom mint body for $2000 or even $3000 I'd be all over that. I'd say a showroom mint car with the entire drivetrain in very good running condition is worth $4000 or more so consider what you think it would cost to get it in very good running condition. Admittedly bargain hunters can sometimes find such cars for less, but on the other end of the spectrum someone recently posted a couple of Eagles in excellent shape at a dealership for the asking price of around $10000 - $11000. One of them sold for near that price.
Having said all that, before you make an offer make sure the body is in as good of shape as you think it is. I thought my SX/4 body was in pretty good shape until I took the plastic body cladding off and pulled up the carpet - all the nasty stuff was hiding under there and it needed a lot more work than it appeared with the car together. Take a pointy object like a scratch awl, and put it all along the inner rocker panels and give it a light hit iwth a hammer. If it goes through the panel is bad, if not it should be fine. Do the same on the floor boards. Unless you've got very good reason to believe otherwise you need to pull the body cladding off and the carpet up to really check body condition.