I have the K1 and it is a great camera, you can have it out in rain and snow (I´m living in north Sweden) without problem. You can also watch this comparison between Nikon D850 and Pentax K1. https://www.youtube.com/watch?time_continue=217&v=ngjFgTQW1Ik
I use this camera with the new 24-70, 70-200 and 150-450mm, 31 and 77mm limited 100mm macro, Samyang 24mm tilt/shift, laowa 15mm macro shift, Pentax DA* 60-250, but also older lenses like Pentax 50mm F1,2, Pentax 85mm soft, and Takumar 85mm F1,8 for example. I have at least 60 lenses between 4,5mm and up to 600mm that I can use so there are a lot of lenses you are able to buy to this camera and all lenses work with image stabilization.

On the K1: 50mm F 1,2 / Pentax-A* 300mm F2.8 / Pentax 85mm F2.2 Soft / Takumar 85mm F1,8 .and Kiron 105mm macro F2,8. On 645: Pentax 67 75mm F2,8 and 105mm F2,4. I have a lot more testing to do, most of m42-lenses I have not tried yet. A lens I have used on APS-C is Meyer Gorlitz Orestor 135mm F2,8. Very nice bokeh with 22 blades. Must try it on the K1.

My favorite SLR:s are Pentax LX and Pentax 67II, my favorite DSLR:s are Pentax K1 and Pentax 645Z. Some of the 67-lenses are fantastic with the 645Z and some of the old Takumar/Pentax-lenses are great with the K1.

You are absolutly right. And there is a lot of very good old primes to use.

Sorry, my fault. With zooms you are right. Personally I don´t use old zooms.

It is no pain to change focal length because the camera asks automaticly when you put an old lens on. It takes a few seconds to chose the right focal length.

Also IBIS works differently with different focal lengths. I´m using Pentax and with new lenses the camera changes the IBIS-function automaticly, but with old lenses I have to tell the camera what focal length the lens have so the IBIS-function will work perfect for that lens. That is why I can use my 300mm Tair Grand Prix from 1958 with the same shake reduction as my modern lenses.

On Pentax you can use all k-mount lenses (all brands but take care with Ricoh k-mount, some of them is not compatible) and m42 lenses from all brands. And every lens will have shake reduction, even the oldest m42-lenses. I have about 80 lenses that works fine on my Pentax K3.

Was it not (or really is it not) a nice camera. I have it with 18, 24 (which is the normal lens), 50, 70mm and zoom 20-40mm. The zoom is not as great as the primes, but they are really high class lenses. I don´t have the 18mm pan focus and the 1,7x extender made by Soligor. I can use the lenses on my Pentax Q7 (with adapter) but just wide open, but because I just use the center of the lenses on the Q7 they are very sharp wide open (f 2.8).

I had to think a little about Virginia, but I´ve got it. Have to check if it is possible to get 110-film in Sweden first. I made a poster from this camera 2001

Pentax auto 110 on analog, it´s a lovely camera with great lenses, but I don´t think you can buy 110-film today. On digital Pentax Q7, also a lovely little camera that shoots in raw-format also with great lenses.

I think you will be happy whatever you choose because all cameramakers make good cameras, but for me it´s Pentax. On aps-c I have lenses from 4,5mm (circular fisheye) to 600mm, about 80 lenses in total, and shake reduction works with all of them even on my oldest lens, a Tair-3 300mm Grand Prix Brussels 1958. To my K1 I have about 60 lenses that work fine, of course also with shake reduktion, between 14 and 600mm. But, as I said in the beginning, all cameramakers make good cameras today.

On aps-c and full frame (Pentax) I am using a Kiron macro rather often, a beutiful lens, and on Pentax 645Z I use Pentax 120 mm macro, but not very often.

Sigma 4,5mm circular fisheye for Pentax

As a lot of people has said here all k-mount lenses works fine on Pentax full frame and aps-c bodies and m42 lenses works with an adapter. Shake reduction works with all lenses regardless their age. With manual focus lenses you can set the body to auto focus, push the trigger down, turn the focus ring on the lens and the camera will take the picture when the center focus point is in focus. Usually I continue to turn the focusing ring a little further and then turn it back and get two pictures taken, one from both sides of focus. And if you have the Pentax 1.7x AF adapter you´ll get autofocus with all manual focused lenses.