I sail a 1977 P30 and looked at several others prior to purchasing Grey Goose. Common issues that I saw with other P30s included wet decks (esp. around lifeline stanchions and the bow rail), leaks near the chainplates and other common issues that you're likely to see looking at any boats of this vintage. One issue I did see a few times that bugged me... the deck-stepped mast is supported below by an oak compression post that goes into the bilge (such that it is). On several boats, there was some 'dishing' on the deck around the mast step. Going below, the base of the compression post had evidence of some rot at the base and was definitely wet. Grey Goose has no such dishing and the post was dry -- as a preventative measure, I glassed in the base to keep it that way. Some also exhibit some swirl patterns in the non-skid, but that's purely cosmetic and easy to take care of yourself. I only looked at one with a Faryman diesel and walked away for other reasons -- ours has a more popular Atomic 4, which has proven very reliable.
Sailing qualities... Do your homework and you'll read that P30s are a bit 'tender' (i.e. quick to heel), a characteristic I would agree with though they stiffen up after 25 degrees or so. Overall, we're very happy -- we do some racing and coastal cruising and she feels pretty solid in a blow. Overnights are fine with my wife and two daughters. Ours has wheel steering (Edson had a conversion kit available), which is nice but admittedly crowds the cockpit a bit. I single-hand quite a bit and find it very doable, though no special modifications have been made.
Practical Sailor did a review a year or two ago which you might want to look up.