The K-5 would be a good choice, but one in good condition might be hard to find at such a low price. When it appeared, it was hailed for bringing remarkable improvement in lower noise at higher ISO without significant loss of image sharpness, compared with previous models. Same could be said about price for most of the other suggestions. The K-5, as well as the prior K-7, K20D, and K10D were flagship models, and will still likely command prices above $100 in good condition. You would want a camera that will be dependable for a long haul. But if you go too far back you lose some features, build, and imaging quality.
I'd say the K200D might be available in good shape for that price. It is well above the K100D for image quality, and is of much better build. Like those flagship models, it has WR construction with weather-resistant seals. After shooting with the K100D for a few years, I found the K200D brought me superior imaging along with other improvements. It is very decent regarding noise at higher ISO. Not as good as a K-5, but still quite decent. Its ISO setting limit tops out at 1600, and not bad even at that level. It has better in-camera imaging adjustments than the previous K10D, and is also better for highest-quality JPEG images right out of the camera- just be sure to go into the Custom Image menus to set up "Fine Sharpening". But that is for more general use, however, as this degree of detail might not be as desirable for portrait use. I believe there is a Portrait category available as a choice in the Custom Image settings, where sharpening can be left as is. This setting will also modify the color palette to be more pleasing for portraiture.
---------- Post added 03-06-23 at 05:12 PM ----------
Originally posted by Tri-X 400 Bright viewfinder is very important
This makes it more difficult to stay at $100!! Then for cameras this old, you might need to stick with flagship models. A K-r would not make it, and maybe not even a K200D. These use the less-expensive typical mirror-prism design. You might then find the K10D to be the ticket to your success. Or if you get very lucky, you might find a good K20D at that price.
These days, and for a number of years now, Pentax has been THE brand offering pro-level viewfinders, along with WR construction, in entry-level models at very reasonable prices!
One of our regularly-posting technicians, also from Germany, going by the name "photogem", would probably advise that you look for a more recent non-flagship model having these attributes, such as a K-30, K-50 or even a K-S2 (a great compact Pentax DSLR with many desirable features) but having no aperture control due to a faulty solenoid that is susceptible to failure in these models. These are often sold at very cheap prices. Then you could obtain a good used solenoid that is not susceptible to this problem, taken from an older non-functioning Pentax camera model, and use it to replace the bad solenoid in the very fine camera you've obtained at such cheap price. You have to be willing to take on this task, and learn the procedure to do it. If so, you could wind up with quite a bargain. Whether you'd be over $100 then- maybe, maybe not.
If you get a camera with this defect, you could otherwise use it as is, but the lens would remain wide open- or you could obtain a Pentax lens with PLM technology, which does not depend on the solenoid for setting aperture. But there are only a few of these lenses so far.