440 $ Rebuildable worth?

I have a friend with a few 440's. We have been talking and I may be able to pick one up for the Duster.
Questions

What is a rebuildable 440 worth? What do I offer, without insulting him, and still get a good deal?

Are there any 440's that I need to stay away from? Or are all of them good? Any bad years?

Where can I find info on casting numbers? years?

Thanks