OK, here are some pointers. Learn enough gunmaker's names to form a basic idea of what the gun is based on the name. Learn to spot deceptive names. Learn enough about proof marks to confirm/refute the maker's name. Learn to sort maker's name, original quality, and current condition. Locate a good gunsmith you can consult on any proposed purchase. Develop an understanding of what you will get for which price. Learn the difference between functional issues and cosmetic issues. Learn what the gunsmith will charge you for the various repairs and services an old gun is likely to need. OK, that is a start.
Second, put your wallet away until you learn some of these things. Alternative, find a dealer who already knows such things and who will treat you fairly. Right now, you would be much better off to go to a reliable dealer and say, "I have $XXX to spend on a hammer gun for CAS shooting. I want #1 a safe gun, #2 a functionally reliable gun, #3 a gun that is not likely to break in the immediate future, and #4 I'll worry about cosmetics later and as I learn more about old guns.
If you want to learn, hang around here and ask questions. Ask something like, "How would you guys rate the above three guns?"
