I was in the same shoes as you. I own a K100D and last winter I considered to upgrade to a newer Pentax SLR. I read a lot of comparison about K5-IIs and K50 but finally I made my decision: K50 and it was a good decision. It's weather sealed, comfortable to hold, bright led aid in low light condition, li-ion battery lasts for long but you can still use AA batteries etc. The picture quality compared to K100D is much better. I wouldn't recommend the K500 for you. Only a "few" extra bucks and you get a more professional SLR feeling with K50.
K5-IIs cost $500 more than the K50 and the only important difference (for me) is the microphone plug. Even the K50 has a lot of features those I don't/rarely use: artificial horizon, live view.
Last words for m42 lens. I own an old Helios 44-2 F2 58mm with m42 thread and that worked nicely on K100D body in manual mode. Mirror lockup was a cool feature on K100D. But on K50 I can't find mirror lockup function and I have to use m42 lens in AV mode. It's not a problem but I need to get used to it.
Please share your thoughts after you make your decision.
Last edited by everysecondiwaste; 06-15-2014 at 10:55 PM.