For all M42, K mount, and M mount lenses you input the actual focal length of the lens when you mount it on the camera. The focal length of the lens never changes only the field of view through the lens. Using a full frame lens on a APS-C camera is like cropping an image down on your computer.
You can read more about crop factor here ->
The Crop Factor Explained: An Animation - Tutorial Videos | PentaxForums.com
Here you can read and watch a video about using manual lenses on Pentax camera bodies.
How to Use Manual Lenses on Pentax DSLRs - Tutorial Videos | PentaxForums.com
I recommend using a lens with an "A" setting. This way you do not need to enter any focal length information and you can control apeture through the camera. I sold all my M and K lenses years ago. Having the auto apeture is just too handy for me but that is just my opinion. If you do not mind stop down metering the older manual lenses are a great and economical choice.
The K3 has front and rear infrared sensors that allow you to trigger the camera from either in front or behind it. You can pick up an inexpensive IR remote on Amazon for around 10 dollars US.
The older super zooms, 28-200, style did suffer in IQ. Lens coatings and technology has come a long way and the modern ones are definitely better. As for being worth it. It depends on what you expect from them.