Locally, Pentax lenses are kind of hard to buy because nobody is willing to sell them, and the shops that do sell them puts on a unrealistic price tag. But if importing is not a problem, K-mount lenses are plentiful and cheap. You just have to search.
Both the Pentax *ist and the *ist D does NOT have full support for the pre-A series lenses. I suppose you are starting a new system so this should not bother you too much but in case you do end up with a *ist (or the MZ-50/30/60), do note that you cannot use the older manual-focus Pentax lenses.