Hi I have a Pentax K100D (recently bought).
Like yourself wanted to experience(play) with other lenses other than the kit lens 18-55.
Managed to pick up a Tamron 70-210mm from e-bay.
This is not a digital lens but from a 35mm SLR and although heavier than a digital lens, it gives me choice, Pentax DSLR can "step down" to use all previous lenses to fit Pentax cameras, this is why I chose the Pentax. As a beginner I did not want to spend a lot until I knew what I was dealing in.
Anyway I bought the lens for £15 including postage, I have seen many more available usually attached to an old Pentax camera, just make sure it is K, KA fit etc.
see the following link for an example of whats out there.
http://search.ebay.co.uk/search/search.dll?from=R40&_trksid=m37&satitle=pentax++KA+lens
Hope this helps.