I little hard to answer the question without knowing what you'd like to try and shoot. Also the budget you had in mind. There are a number of factors to consider here. This is a great old camera but some day you might switch to a digital if nothing else but for the much lower cost vs film and processing. So when choosing a lens do you want to just start off cheap and see how you do or buy the better lenses that will give auto focus if/when the digital conversion comes.
My first recommendation would be to get a 50mm f1.4 (or 1.7) or the 43mm limited. The 43mm would be the best choice in my opinion. A wonderful piece of glass and considered the true normal lens on a film camera. Added bonus is that it is AF for a future body upgrade. But the 50mm's even though they are manual focus (there is an AF version but the focus ring will drive you nuts on an MF camera) they are great lenses that can be had for around $100 here. And just learn either lens and the camera first. I think it is a mistake to buy 4+ lenses before knowing whether this is for you. If you want more right now I'd look for a SMC Pentax 70-210 zoom either the manual focus or auto focus versions. Again the AF version will be a little harder to work with as the focus ring is not as smooth as the MF designed version. Both are reasonably priced but only found on the used market (Ebay camera shops etc). Last lens I might suggest is a wide angle. A good 28mm f2.8 would do the trick here. Any of the Manual focus lenses will work on a digital just fine if you ever go that route. When selecting a lens, try to look for the ones that have the "A" setting (button) on the aperture ring. This allows the camera more auto exposure functions.
For a little light reading I would highly recommend go buy the books 1)Photography and the Art of Seeing (2004) By Freeman Patterson and 2) Understanding Exposure: Bryan Peterson (2004)
Both are excellent books and although they cover some similar ground, well worth the read. If you get these and read/practice with them and still love photography then it's time to look at other lenses.
For some instant information these 2 sites are excellent:
The Radiant Vista The Luminous Landscape
You'll learn a lot here without spending any money. Also cruise through the forums older threads and you'll find lots here.
Welcome to both the forums and the world of Pentax!
Similar Threads 
