I brought a used, 6 month-old K-5 and owned it for another 6 months and had never had any sort of problem with it.
I've had issues with K-x and K20d not showing the correct battery life and not saving pictures to SD cards, but in most cases, things like this are usually fixed by a firmware update. Also, K-5 has AF adjustment, so in the worst case, you'll have to perform the test and adjust your lens accordingly, which really isn't a problem.
Don't worry so much.
Enjoy your K-5, don't regret purchasing it, and should you have any problems or questions, just post here and I'm sure you'll get what you needed.