"Very lightly used" and 600 hours on a 2 year old diesel are NOT compatible statements. That adds up to almost 6 hours a week of engine use, 52 weeks a year. Since most of us sail more than we motor, and given that the boat must not look heavily used, I'd say that the correct phrase would be "used a whole lot - but lovingly."
According to my mechanic, here in Maine most boats don't accumulate 100 hours per year in our 5 month sailing season. My boat's previous owners used her lightly - and accumulated 560 hours over 19 years.
Is this a reason not to buy this boat? NO!
600 hours on a diesel is nothing, assuming that the oil was changed every 100 hours and before storage. However, if the engine has been run at a low load to charge batteries, especially when new, the cylinder walls may be "glazed".
I've never seen this condition personally, so I can't advise you on it, but I've been lectured on and warned about this by several mechanics over the years. I can't advise as to an engine survey, as I just asked the man who'd been servicing my engine for a decade or so - he said it was fine and I trusted him.
You should have a sailmaker examine the sails and tell you how much life they have left. The cost would be trivial, and given the number of hours on the engine, I'd suspect the sails may have seen considerable use for their age. However, I doubt they're worn out after 2 years.
Lightly used, here in Maine with a 5 month sailing season, my 10 year old jib is about 1/2 done, according to the sailmaker who replaced the sunbrella (which was sun-rotted). My main is also 10 years old, and is essentially new, as it was rarely used.
In the tropics, if sailed regularly all year round, you might need sails every 2 or 3 years. The shape would be going after one.
The "best practice" is to deliver your sails to a local sailmaker each fall for cleaning, checking and storage. He or she will tell you when your sails are ready for replacement.
Others may know of issues, but I know of no model-wide or manufacturer-wide issues with these boats.
This spring, I compared notes with the owner of a recent (2002?) 40 footer, and he had no issues with his boat other than normal wear and tear.
Good Old Boat did an article on the PSC 25, and commented that these boats have generally been free of structural defects (except for early aluminum fuel tanks - which sat in bilgewater and needed replacement every few years).
My boat has been essentially free of structural defects since new (I have the yard records). I had a small delamination of the glass covering on the stringer aft of the fuel tank. The surveyer called it cosmetic. The yard called it trivial and fixed it with ease. The stringer was completely sound. That's it.
After 20 years, and excluding electronics, the only major maintenance items were replacing the fuel tank (because it was 19 years old and I don't trust old aluminum tanks - it wasn't leaking), replacing the glass in the ports due to age-related delmaination of the safety glass, rebedding the dorades and mast pulpits, and replacing the cutlass bearing (which was a big job - the rudder had to be removed first). That's all I had to do.
Obviously, the systems installed in these boats are subject to age and wear, but all the original systems in my boat were top notch, installed properly, and are either still working or gave a normal service life.