I'm not a Pentax user, but you might want to check and see if they offer a lens that might include everything in one. For instace, I have a 28-300 lens for my Canon.
Usually with a lens like that, you give up a few stops in the Aperature range, but for the luxury of not having to change lenses, it more then makes up for it. Being as you'll probably be outside most of the time, a low F-Stop lens probably wont be needed.
Being as the film bodies have dropped so much in price, you could always look for another body so you wouldnt have to change lenses at all. I know I saw some film bodies the other day for $50 in a pawn shop. The guy said he'd make me one hell of a deal because everyone is pawning their film equipment to switch to digital. You might want to look at a few Pawn Shops before you buy anything.
Good Luck, and make sure you post some pictures after you return