Those are F and FA-series autofocus lenses, so for a Pentax DSLR just park the aperture ring in A and leave it there; control the aperture through the camera full-time. Effortless.
However, if you want to do macro with film-era extension tubes, or just have a play around in full manual for curiosity's sake, those aperture rings will come in handy. And then you need
this guide.
Regardless of the field of view a digital camera offers you vs. film, I would use what I had at first (and it covers quite a wide range), and then think about where it wasn't meeting my needs before I rushed out and bought anything else. Uncle Vanya is right about one thing - if you go Pentax digital you will probably eventually want a wide-angle APS-C lens (unless you buy the very expensive K-1, but you already said you didn't want fancy or pricey). You could invest in (say) the 16-85 zoom, which would match up nicely against the wide end of your 80-320.
Be aware that if you get the film camera out of mothballs and fire it up, the newer digital-era DA lenses
will work with it mechanically and electronically, but almost all of the zooms and many of the wide primes will not cover the entire film frame.
I don't own the Kx but I do have a much older *istDS, and that can still do some fantastic work; I see no issue per se in buying older cameras if money is tight. Depending on your budget, though, you might want to go for something newer and much more modern, and much good advice has already been given here. Once you get to the K-5 and the cameras derived from it, however, there is a HUGE leap in capability.
Bear in mind if considering a K-50, that it has a well-known failure mode in the aperture control element, so you may be taking a chance there. I have a K-5, bought new when it was still the best Pentax had, and it still remains a great camera. The -ii and -iis are better, but they weren't enough better for me to buy a whole new camera for the smaller improvements when I'd only just coughed up for the original.
If you get a K-5iis in good shape, you may never need to buy another digital camera again as long as it lives.