Simply put, don't stress over "equivalence" as they are rather pointless these days, unless you are using two cameras with different film/sensor size and you want to capture the exact same.. Field of view, or depth of field, or some other thing that you want to keep constant.
A 50mm from 1970s will give you the same FoV as the latest DA 50mm f1.8. Just the film plane is slightly smaller than the 70s film. So if you compare a modern APS-C sensor with film, it appears that the edges are cut off and effectively you have a relatively reduced FoV. You can even get a medium format 50mm and adapt it on your camera. It will just project a large image circle that the sensor won't record, and so the photo will look like other 50mm lenses.
Unless you shot a lot of film, you shouldn't worry about equivalences. Just get the lenses you like. If you buy old lenses, I suggest you get a nice hood along with them, because lens coatings back then were not as great and a lens hood can really improve older lens' IQ. And I suggest you read up on Catch in focus - a great Pentax feature that gives you some AF-like functionality to manual lenses.