i have a question about a used bassoon it is missing one screw on a gaurd and and two on another an just a few cosmetic damage oh and is it better to use a metal bocal or a plastic one

Refitting key guards should be very cheap. You don't specify what the cosmetic problems are - is it just dirty? Is the plating on the keys worn? Is the body finish in a bad way? Also what make, age is the bassoon? What price is being asked for it? That can dictate what amount of work is reasonable to do on it.

I have never heard of plastic bocals being made - please post a link if you have seen adverts for some.