I have a few M42 Pentaxes and lenses, and have used them extensively for some years. The bodies are of very high mechanical quality, but after 40 or more years, many of them perhaps sitting unused in a drawer (or worse: an attic or a cellar), they will most likely need a CLA. Other than the need for a CLA, the main issues with these cameras are that the viewfinder is a litte dim, which can be a problem with slower lenses, and that the meter in the Spotmatic, at least in my view, isn't very useful, even when working correctly. I usually prefer using a handheld meter anyway, so the meter issue doesn't bother me.
When it comes to lenses, they are generally good, but a lot happened in lens design during the 60's. Some focal lenghts came in new versions during that time, and especially some of the wide angle lenses were considerably improved (f. ex. the 28mm 3.5 and 35mm 2.0), although some were very good even though they were early designs, such as the 35mm 3.5. When in doubt, try to get the later versions.
The Super-Multi-Coated lenses are obviously better when it comes flare and such, but I haven't noticed any particular problems with the single coated lenses. It's always a good idea to use a hood anyway.