For that particular 280, just from the few pics, IMO I wouldn't offer more than 1500 tops if all the planets were in line. And that's if if ran and could drive it home. Mainly because you would spend at least 7K just to get it safely back on the road, plus whatever else it will require, interior suspension, brakes. The list goes on…… and then there's body and paint.
As far a looking for a project Z (they are all projects by the way, even if they are "finished") a lot depends on what year and condition. Series one 240s all original with matching #'s currently go for the most money. Everything after that kind of works on a sliding scale. 260's seem to be worth less even though they were only sold in the US for one year. There are still quite a few 280's out there that are in fair shape. ZX's can be even cheaper but the prices have climbed on all the the Z's lately. I don't know where you are posting from but location also makes a difference. West coast cars seem to be reasonable to us east coast folks just for the rust factor alone. Wheee! just gave you some ball park figures that seem about right.
Keeping those #'s in mind you can approach your future Z with two thoughts in mind. You can bring one back from the crusher and get the satisfaction of saving a Z and it will become "your" baby. A lot of us have done that. Or you could search for one that is mostly done. You'll still find things to fix/restore but the work will be significantly less. There are quite a few I've seen in great condition around the 8-15K range that I know the builder/restorer are losing money on. Much like if I were to have to sell mine right now .
Spend your conscience and what you are comfortable with wallet wise. If you get infected like us, it won't matter in the long run.