"Dedicated" just means the camera and flash can communicate with each other and (to one degree or another) change settings. It has no direct relationship to TTL or P-TTL, although those can be a part of it. The K100D and the K10D are alike in that they both use P-TTL and can not make use of TTL, by the way.
With "auto" mode flash units, neither TTL nor P-TTL have anything whatsoever to do with anything at all. Pretend neither of them ever existed. You manually set the shutter speed to 1/180 or slower, then set the aperture to the setting indicated by the flash unit. Then you just clickety-clickety-click your pictures and the flash unit automatically controls the light output for correct exposures.
Here is a photo of the back of a dedicated Pentax AF200T flash:
Notice that the ASA (ISO) dial is set to 200 and the mode selection switch is set to "green". Now look above the switch and you see that for any distance covered by the green scale (0.5~3.5 meters) the flash will automatically provide enough light for an exposure at f8.
So all you would do in this situation is set your shutter for 1/180 or slower, set your aperture to f8, and shoot pictures. In certain camera modes, this dedicated Pentax flash unit can automatically set the aperture for you, and will even set your shutter speed. If you change the ISO on your camera, it will even automatically adjust for that.
The "TTL Auto" mode is, of course, useless on the K10D and the K100D. Sort of like tits on a boar hog.....not doing me any good, but not hurting anything by being there either.
So long as you buy the 285HV new, you should be alright. There are several vendors who sell them and they run about $85, I think. Or if you decide to eBay around for alternative models, just be sure to
look here first to make sure it doesn't have a trigger voltage that will fry your camera. Not all old flashes are dangerous. I use the Vivitar 2500 I bought new in 1982 (around 10v) but can not use a similar vintage Vivitar 2000, since it has a trigger voltage of well over 200v.