Are you sure it's a 17? The first Hobie 17's weren't produced until 1986, so he's either telling you the wrong year or the wrong model.
I would inspect all repairs by visibly looking for cracking and inspecting the general workmanship. Also inspect by pressing hard on the repaired area to see how much flex there is. I would be wary of any soft spot that was simply repaired with a patch. Soft spots are generally the result of the fiberglass hull skins being separated from the foam core which means they need to be re-laminated to the core for proper repair (generally by resin injection).
Other things to look at- definitely inspect the wing sockets for leaks (fill with water and see if they hold). If they leak, you've got a messy repair on your hands that requires the installation of port holes and is a PITA. Also, check the sail. Old mylar sails have a tendency to delaminate which has seriously detrimental effects on the strength and performance of the sail. Last, check mast and crossbars throughly for signs of denting, bending, cracking, and corrosion. Any of which could indicate that the spar needs replacing.
$1100 could be a good price if everything is in reasonable shape, but keep in mind that when you have to start replacing parts, the cost goes way up.