Without knowing what you have or the condition of the marker......I like taking PGP's or any Sheridan apart and putting new seals in myself....because then I know it's done. HOWEVER....you need a special valve tool. A guy here on MCB named Titus has those for a good price. You can track it down in the "Dealers Forum".
I don't really believe in polishing internals. The joys of Sheridan to me are that they just work. A new bolt....maybe. A delrin one (again Titus has some red delrin ones) or a Cooper T bolt if you can find one are always nice. Honestly to me the cool part of a PGP is that bone stock it works.
If you're going to upgrade the one thing I would recommend is a quick change for CO2. Basically it's a sleeve that screws into the fine threads of the co2 tube and then has a big thread knob that threads into it. So instead of 25 small turns it only takes 1 and half.