If you own and work on an old VW bus, then you certainly have: 1) a high tolerance for misery, and 2) little regard for your personal safety, so getting involved with regulator repair and service will be right up your alley!
(I say that as a former owner of a 65 microbus that I rescued from being used as a chicken coop)
You can check the condition of your piston yourself, it's easy. You need a tank, an IP gauge, and a jeweler's loupe. If the regulator does not have any IP creep with a new seat, the piston is fine. If it does creep a little with a new seat, you can carefully inspect the edge with good light and a the jeweler's loupe. Any scratches or flaws on the knife edge at the end of the piston shaft will be easy to see. Your regulator is likely already apart (hence the comment about the older style piston from the shop) so maybe ask if the guy bothered to check the IP before he took it apart, and ask if it was creeping.
There may be other reasons to look for a different MK5, though. IMO, the later models with 5 LP ports and 2 modern-sized (7/16) HP ports are the most desirable. They frequently have had the brass turret retainer replaced with SS as well. You could post a photo of your MK5 and several folks on this forum can tell you what generation it is.
A good way to start with DIY regulator repair is to get the Vance Harlow book on regulator maintenance and repair. It's not unlike the old John Muir VW idiots guide, but without the great drawings unfortunately. Between the Harlow book and the help of some fairly knowledgeable regulars on this forum, you'll find it very easy and inexpensive to get into servicing your own MK5. Best of all, and to me this is a REAL plus, you will never have to listen to some BS about old regulators from a dive shop again. Some dive shops have excellent techs, many do not, but ALL of them are in the business of selling new regulators.
