The answer to this question covers a lot of ground. I can only share what I know, but I do have a few classic lenses. Most of them were bought when film was still the only game in town. Unfortunately I don't have a K1, but here goes.
M 20mm f/4: should be wonderful on full frame. It certain was on film. Getting the composition right can be tricky with ultra wides such as this but that is how it goes.
M 28mm f/2.8: very nice. Some say that the f/3.5 is sharper but I don't have one.
M 50mm f/1.4, f/1.7 and f2.0: they are all the same and all different. Get them all and have fun.
A 50mm f/2.8 macro: just wonderful. Goes up to half life size magnification for close up work and very sharp for other work.
M 100mm f/2.8: compact and very useful. It got a lot of use in my film days but these days it is overshadowed by my Rokinon 85mm f/1.4 (try that one as well).
M 200mm f/4: compact and very useful. Probably the one I used most in the film era. Still cuts it in the modern era.
K 200mm f/2.5: big and heavy, tricky to focus. Get it right and it will deliver.
A 400mm f/5.6: just wonderful. I love mine. Modern equivalents are said to be better but the results I get from mine are just fine. I see any reason to get something newer as one doesn't use a lens like this on a daily basis.
If you come across a Ricoh XR lens from yesteryear, grab it. I have a 50mm f/2 that I picked up for cheap. The 50mm f/2.2 is
notorious famous for being very bad. I am looking out for one.