1968 5513 should have a matte black dial, with feet first depth rating.
Bracelet would be 9135 (early folded with flip lock, non solid link)
or perhaps even rivet 7206. I am not 1oo per cent sure on bracelet issue.
Regarding price. Can't even begin to help you. Just study the market for a long time. Make no quick moves. Don't be afraid to pay more than the usual market price to get an unusually good example. Don' t buy junk, even if "cheap" as it
will inevitably be money down the drain.
Don't let on you are only interested in 68 or you will wind up paying even more.