Ok, so here would be my list of recommendations for manual focus K mount lenses that are worth buying.... (In order of cheapness)
135mm f2.8 takumar. Cheaper than the pentax variant, it doesn't have the 'a' contacts, but is a good lens. The focal length is a bit too long to be everyday useful.
Pentax-m f2.8/3.5. They are ok, but nothing that will blow you away (they are at a focal length where you want bokeh on dslr, but they really aren't designed for that)
Pentax-m 50mm f1.4, or pentax-a 50mm f1.7. They should be about the same price. The 1.4 is better, but with the 'a' contact 1.4 is pretty expensive. The da 50mm f1.8 can be found fairly cheap 2nd hand, which is probably a better bet (wins on the usability front).
Pentax 55mm f1.8 'k'. Nice lens, probably better to buy the da50 though.
Pentax-m 100mm f2.8 non-macro, or 85mm f2.0/f1.8. You'll pay a premium due to the rarity of them, but they are very nice!

. (The 85mm lenses can be very expensive)
Pentax 50mm or 100mm macros. Tend be be pricey, autofocus versions can be found for almost the same price.
---------- Post added 08-28-14 at 11:31 PM ----------
Originally posted by when-daybreaks
Thank you older lens can be yellowed right? That's one thing to look out for. . And scratches and mold and oil too.
They can be yellowed, but that's less of an issue compared to:
* lens has been dropped once before, and is now decentered - the most common problem.
* smc coating has been removed from front element due to over zealous cleaning - flares are common.
* fungus - can be a costly repair, and may have damaged coatings (so a sale may list no-fungus, but the damage may have already been done from a previous infestation).
In my experience, scratches and oil are less common in your average eBay sale (sellers tend to list those faults), but ymmv.
---------- Post added 08-28-14 at 11:35 PM ----------
Just buy a da 35mm f2.4 as your first lens, it's unlikely you'll find a bad second hand copy (they were being given away with k-3's a little while ago, so I've seen a lot on sale from 'pro' users who already have the limited version)