Heck, I'd say you are lucky to have a shop that'll field strip it for you. All the local places won't do it, or even let you do it, even if you know how. I only buy brand new stuff from the local shops. If I go to my fav place in Utah, they strip it, will let you shoot it, hell, I bet they'd let you clean it! hehehe.
Buying guns is all about how valuable it is to you. I bought a P99 for $400 used. Had to replace a recoil spring, shoots like a champ. It was filthy as heck.
To be honest, if it's a gun of new manufacture, you'd really have to abuse it to make any wear and tear cause any sort of malfunctions. That's not to be confused with "dirty". A dirty gun can cause malfunctions, but as has been pointed out, a simple cleaning will take care of that. I tend to look at the exterior finish and the barrel. That's it. If a gun is beat to heck on the outside, it probably wasn't maintained well on the inside.