Rolex 6263 non-Daytona sigma - should I buy?

[Jay Kay]

A contact has found me a 6263 c1974 with a pre-Daytona sigma dial. It has a 727 movement in working order. The case has been changed in the '80s with a c1975 case, now with a bit of pitting. No papers but there is a box.

Probably my grail watch or very nearly. I'm concerned about if an exchange case seriously affects the value or abilty to resell, if ever, and if the lack of papers is a major problem. Is it a Frankenwatch in your opinion, with little value, or is it a serious purchase opportunity?

Questions guys: value you'd pay for the watch and should I buy or hold out for another watch with original case & papers?

Thanks.