cracks on eck
Are you referring to "crazing" in the gelcoat around the mast....or is there very definite cracks visible in the deck/coachroof that you are seeing that extend into the FRP?
If you are not seeing actual cracks in the FRP....then more than likely it is only gel coat cracking and that is a repaid that you can do your self. Its not that difficult. I have no idea how much it would cost as that would be subject to a quote/estimate from a professional.
If you are serious about buying the boat - hire a surveyor and get his opinion. It will be well worth it and he can check for things that you may or may not spot. Also, many insurance companies require surveys before binding over a boat on a policy.