Still available second hand (sorry pre-used) is the f/4 SMC Pentax A one touch zoom. it is manual focus, but simply a cracking lens with a nice out of focus soft transition. It is also f/4 over the full zoom range and gives 105 to 315 equivalent on the K10D / Samsung body. I have been using mine for years and it still pleases me. It takes a 58mm filter.

The one touch, by the way, means one large control ring twist to focus and slide to zoom.