I can't see that firing modern 318s in the gun would be a problem.
I do have some original 318 ammo and it does indeed have quite a sharp shoulder almost like an A. Improved case.
A look at the fired cases would be nice as mentioned above. As long as they are the same in both chambers and fired cases from each will chamber in the other, a neck sizing is all that would be needed when reloading.

Can't say a lot about the value but I think the sum being batted around is right around what I would expect without really knowing what the bores look like and how it checks out overall.