K mount should fit OK. It will work in Manual and Aperture Priority modes.
The only problem with the Pentax backwards compatibility is the price of secondhand lenses - especially the 50mm primes which are getting extortionate. The FA lenses from the 80s and 90s are superb and some of the zooms are cheap(ish). If you go to the Pentax forums there is a lens guide
HERE.
Don't be fooled by the DCM reviews of the 18-55 Pentax kit lens - the French Camera mags rate it as good as the Nikon one and way ahead of the Canon offering - French mags use lab bench tests to rate kit, not the subjective views of journalists.
Choosing a camera system is very hard, because they are all good. It has to be down to how the body feels in your hands and do the ergonomics suit you.
I'm happy with my Pentaxes, but I am sure I would be just as happy with similar offerings from the other big names. If they weren't so much more expensive
