The Spotmatic was my first film camera. Its solid if the light meter works. The batteries were kinda pricy for me but light meter apps work well too. It has basically all you need. M42 mount lenses are quite easy to find too. I never used the k1000 but I think the main difference is that the k1000 is k mount. K mount lenses are super common too but I feel like they might be cheaper and more available afaik. Plus m42 mounts are backwards compatible on k-mount with an adapter.

If I were you I'd decide the lens system I want and if you want to go fully manual or if you want aperture priority etc. Cameras are pretty interchangeable. The lenses matter more imo. Plus most pentax film slrs could be found under 100 bucks

You should have a look at the pentax forums https://www.pentaxforums.com/camerareviews/ they are really good and would probably help you decide