Unless you absolutley want a Pentax camera, i would buy a used Canon 5d classic. 12.7 mp are equivalent to most 35mm film, yes low iso/grain 35mm film and a high quality scan can yield more resolution, but in practical terms you will most often get around 12mp. Other advantages are better build quality, better viewfinder and of course full frame. As for the age of the camera: i think it is not much of a problem, it has pro build and a camera with low shutter count will not have the sensor destroyed in the near future. Pair it with the 50 1.8 and one of the early eos wideangle primes like the 24 2.8 and you will get a lot of image quality for your money. You could also buy a Tamron 28-75 2.8 if you desire a zoom. Of course it is perfectly possible to use the camera with your mf primes via an adapter.