You can use any pentax lens ever made... I use a pentax m 120/2.8 which makes a great telephoto for the DL... I would like to get the new 40mm pancake lens which is made for digital cameras... the 12-24 sounds interesting but don't know anything about it (how big, etc)

Since you have the 18-55 already, I suggest the DA 50-200 from pentax. It is $199 after $50 dollar rebate at most camera stores(could be cheaper online). This will give you pretty good range with 2 good quality lightweight lenses. I, too, was sick of the p&s and bought a body only ist DL and originally purchased a sigma 24-135(heavy). If I had the money my choice would be sigma's new 17-70 2.8 macro and the APO 70-300 (around $500 for both).

Well if you don't want to "break the bank"
at first to start off maybe get something like a Pentax Lense 28-200 or 28-300 there abouts.
You don't have to change lenses it will do a reasonable variety of shooting
There may be varying degrees of "fast" lense availble and some of the cheaper lenses may be around high 5's or 6's on the maximum focal end.
Of course faster lenses around high 2's or low 3's are more desirable and money.
I would watch the good used market.