I've used three different flashes on my Pentax DSLRs.
First was my old Sunpak auto 144 D that I had bought back in my K1000 days. It's very simple, just a swiveling head and some basic adjustments where you set the ISO and you can choose a couple different levels depending on the distance you need, it tells you the aperture to use - so I used it in manual mode with good results. Very basic but it
does get you much better results than the built-in flash.
Next was my Pentax AF-360FGZ. This was their lower-end flash at the time (they've since released a real cheap ~$100 flash) but has most of the features of the larger one, just not quite as bright as no swivel head. It zooms, does P-TTL metering, focus assist grid, wireless triggering with K10D/K20D/K200D's built-in flash, built-in diffuser and catchlight, and a bunch of other features.
Finally, I recently got a Sunpak auto 433 D in a local buy along with some other equipment. It's an older but high-end flash, and it does swivel. It's got more options for control and power level. The guide number is virtually the same as the Pentax so it's my go-to flash if I need the ability to swivel.
If you just want to get started for low buck, I would seriously consider purchasing an older flash and see how that works for you. If you find yourself using it a bit and want more features, you can then upgrade to a higher-end model. You can pick up a decent used flash for very, very cheap over on eBay.
One concern with older flashes is voltage, though. There's a big list
here of how much voltage they use. Too high and they can damage your camera. 6v or less is definitely safe and I've heard that Pentax has told people that it can handle up to 30v. I would lean towards making sure it's fairly low, say under 12v.
Also, if/when you upgrade to a "proper" flash (Pentax or Metz or Sigma or whoever), you can then get a cheap Cactus wireless triggering setup to fire multiple flashes at once. Mine just arrived yesterday... they are only safe up to 12v, BTW.