After buying DSLR I went a bit Ebay crazy for old P/K and M42 lenses. It turns out this is not the best idea if you dont really know what you are buying, which i did not. I ended up with a few cheap lenses, many of which had some sort of mechanical issue.
I realised I was wasting money. I did some more searching around and discovered PentaxForums.com. This site provides some good insight into which lenses are worthwhile, and the trading section provides a good place to acquire all sorts of Pentax gear. The general consensus is that SMC (super multi coated) Pentax lenses are the pick for build quality, colour and contrast.
I went back to Ebay, and kept a good lookout for some decent SMC Pentax lenses. I managed to get hold of a Pentax SMC -M- 28mm f2.8 lens for a reasonable price. It is quite well used but it is lovely and sharp, and a great focal length on a digital APS-C sensor. Added to this were the two lenses I had borrowed from Dad's old K1000 kit. A Pentax SMC -M- 50mm f2.0, and Tokina 100-300mm which has some hazing in the rear elements, and the one good lens from my previous Ebay purchases, a Hanimar 135mm f2.8 M42 which seems to be quite good.
Later I got hold of a Pentax SMC -F- 35-70mm f3.5-4.5 on Ebay for a cheap bid. This is a well regarded older Pentax AF lens, it is quite small, sharp and cheap. Then I went looking for a better 50mm. I found a Pentax SMC -M- 50mm f1.7 (the sharpest of the Pentax 50mm's) after putting in a low bid. Soon after I was able to upgrade that to a Pentax SMC -A- 50mm f1.7 (has camera controlled aperture) for about the same price as the -M- cost me. I sold the -M- soon after.
Missing from my lens range was a telephoto lens. I bought a Tokina RMC 70-210 f3.5 manual focus lens from PentaxForums for a reasonable price. It seemed like a good cheap manual focus alternative while I waited to be able to afford something better. I took some very nice shots with this lens.