A year+ ago I was in the same boat as you, keen to explore this scene but otherwise no clue. After a while (and lots of browsing) I focussed on PK (due to owning a Pentax ME super for many years and a couple of PK lenses), some of the general topography became clearer:
- vintage Pentax OEM (Asahi, takumar etc) MF generally all good. Distinction between K mount and M42 in the technicalities of how they work on camera. PK easier but M42 more versatile (Av mode).
- vintage primes generally to be preferred to zooms. Specifically, the older the zooms the less likely they are to be any good.
- the multifarious generic brands typically dating from the 70's - 80's boom period of Japanese lens production are typically not as desirable and these days I doubt I would bother with any except where I have specific information/insight as to quality. Some of the more common names are Photax, Hanimex, Paragon, Optex, Soligor, Vivitar, Makinon, Miranda.... but there are absolutely loads of different names.
- I got to know the names of the main manufacturers: Tamron, Tokina, Cosina, Kiron (Kino Optical), Sigma (these all produce/produced lenses under their own name/brand), Komine, Hoya, and started picking up on how to id lenses under other marques (vivitar is a classic example) that they had made. mflenses forum is the best source of that sort of info.
- Russian/former Soviet bloc lenses tend to be good (very good!). Lots of classic primes often based on classic zeiss designs. Tair, Helios, Jupiter, Mir, Meyer Optik, Pentacon..... et al.
- Other companies got in on the slr boom with their own PK mount cameras (Chinon, Cosina, Ricoh) and generally their lenses are pretty good, particularly Cosina
- Tamron have a particular cachet with their (now discontinued) adaptall range. Swappable mount means they can be mounted on any camera (pretty much).
Then the other perspective is what sort of lenses to go for. I bought and sold a variety of mostly zoom lenses before focussing, correctly IMO, towards prime lenses that offered clear/specific advantages over the kit lens. First (and best) example: a decently fast "nifty fifty" (update - I c you have one ). One of the classic f1.4/1.7/1.8 50mm or 55mm pentax lenses will give you ~ 3- to 4 stops advantage over the kit lens at 50/55mm, better IQ and the creative possibilities that the range of depth of field offers.
Other suggestions:
- a classic 135mm eg a sonnar, lots of quality options in this focal length.
- a quality macro lens. some are better suited to general use than others, all are sharp.
But beware, lens buying addiction looms....
You can check out my overview of the tamron adaptalls
here