In the interest of full disclosure, I must admit that I am vehemently opposed to zoom lenses of any type, and my number one recommendation to you would be the 100mm f/2.8 pentax prime.
With that said, if you do choose a zoom, don't choose one where you do the zooming by sliding the lens barrel in and out like a pump action shotgun. The 'dust pump' design does just what its derogatory nickname indicates, it draws dust inside the lens body. The design where you rotate a ring to adjust the zoom is far better. Also, be sure to get one with a fixed aperture throughout the range of focal lengths, and be willing to accept a relatively small maximum aperture, as well as a decrease in sharpness compared to primes. Another thing to consider is that since old pentax lenses work with current pentax dslrs, and (my guess) 8 out of 10 dslr users are mainly zoom lens users, the pentax zoom prices are pretty high right now. Something like an 85 or a 100 mm prime would definitely cover, with a minimal amount of foot zooming, most, if not all, of your short/medium telephoto needs, especially if you use a fine grained film that can stand up to a bit of cropping if necessary. In addition, I would recommend sticking with either pentax or ricoh lenses. Loads of companies made lenses for the K mount, some are great, and some are pretty horrible, but the learning curve for figuring out which is which beforehand is much steeper than sticking with the main brand lenses, because a lot of the time, different lenses under the same brand (vivitar comes to mind) were actually manufactured under contract by different companies at different times.