You're not going far enough. Not only can you use any K-mount lens you can also use M42 lenses aka Pentax Screw Mount.
A few years ago, before I go the K10D I was buying quality used lenses on FleaBay. And they were cheap. The K10 came around and they started going up. After the K20 they skyrocketed from where they were. Lenses I bought for $10 are going for over $100 now.
Hoya was the last before Ricoh.