For a Strobist setup, you need very little.
- A compact light stand
- A compact umbrella,
- Umbrella adapter for the light stand
- A relatively inexpensive manual flash.
- Some method of triggering the flash off camera (sync cord or wireless)
The flash must have the ability to change output power. I use a Vivitar 285hv - about US$90.00 at B&H. The stand and umbrella should run about another $70 or so. The trigger will depend on how much you want to spend. I use Pocket Wizards. They are the most expensive option, but I use them in my studio, so they serve double duty. There are some cheaper ebay options, I think Cactus brand comes to mind. I don't know how well they work. Lastly you could get a hotshoe to PC adpater and a long sync cord (about $30.00).
If you have a K10D or K20D, you can fire an off camera Pentax P-TTL compatible flash wirelessly using the camera's built in flash, in commander mode. K100D-K200D you would need a 2nd compatible flash on the camera to serve as the commander. In either case, you would not need any other trigger device.
Personally, I use the strobist method, with the manual flashes. I find it easier to set up and use. Once you gain some experience using the gear, it becomes very easy to manually set exposure. Setting the proper exposure becomes intuitive.
I will use P-TTL with on camera flash, well sort of. I put the flash in P-TTL mode, and the camera in manual. It forces the flash to adjust power rather than the camera to change settings. One other thing, when using on camera flash, you should try always to either bounce the flash off a ceiling or wall, or use some kind of light modifier, like a softbox or diffuser. Direct flash is very harsh and unflattering, using a modifier or bouncing also will minimize or eliminate red eye/devil eye(animal version of red eye).
Just my 2 cents, hope it helps.