What I would pay is as follows
Motor is stuck, frozen, rusted, probably not more than $100.00
Motor turns easily but it cannot be demonstrated that it runs, up to about $300.
Motor can be demonstrated running and does seem to be in good condition, up to about $600
Freshly rebuilt with documentation to prove it, depends on just what has been done, but over $1000.
I'd be careful with the $1200 312. If he has documentation for the crank work, that's a plus. I have a feeling that he cannot prove that the heads were rebuilt. Since the engine is obviously apart, you might be able to confirm that when you see them. Ran when pulled means nothing since it has been taken apart but you should be able to assess the condition of the parts better. In my opinion, "ran when pulled" means nothing, period. Make sure it is complete. This one is worth a look. Just be cautious. Take a mic set or caliper along to check things the best you can.
The 292 is worth it for all that you get, particularly if the rebuilt c4 claim can be proven but nothing is said of the 292, which seems most important to you.
Lawrenceville, GA