I might go a little more recent unless you already have a good selection of M42 mount lenses. Look for a Pentax KX or K2, the metering on these is much more accurate and it's open aperture too. The K2 was their first production model with an electronically controlled metal shutter so the speeds tend to stay accurate and it makes a lovely solid "crump" noise when you press the button.
I have a couple of SPs and while the build quality is first rate they definitely need attention after so many years. My KX on the other hand has had nothing but a clean, new light seals/mirror foam and some tweaking to make the aperture display window work. It still seems accurate enough for slide film at all shutter speeds, judging by the rolls I've had back from it. The K2 has had a bit more work to straighten a bent hotshoe (turned out the top plate was dinged, so took that off and beat the dent out) and replace a broken spring intended to hold the winder lever against the body but the meter and shutter both seem very accurate.
I have seen a good number of recent threads that people are so concerned with the noises an SLR makes. Unless it means something is wrong like the shutter is going etc.. if one is concerned about mirror slap and shutter noise then should really look at rangefinder instead.
Well I did just that in the late 1980's and bought an M3 Leica. However I had by moved to K mount cameras a few years befor and my M series Pentax cameras were very much quieter than my Spotmatics.
I didn't change becasuse of the shutter noise but it's a valid observation which people ask about.
I've now owned 5-6 different Spotmatic bodies over the past few years with most being from thrift stores, and some being hand-me-downs/gifts. They've all been accurate enough to run slides through, and have matched my EOS 3/DSLR bodies meters close enough to not be an issue. I'd recommend them but only if you are comfortable reducing flare in your own technique/methodology. The lenses you can run are definitely not up to modern standards if you don't know some techniques to help out.
The longer lenses are definitely not up to modern standards in IQ either.
In my experience you can expect the shutter to run a bit slow. Capping and dragging is also fairly common, but is sometimes simply a symptom of a lack of use and will clear after some exercise. Whatever you do, do not try to "clean" the shutter by dripping lighter fluid into the camera, you will cause the shutter curtains to come unglued from their spindles/drums. This sort of thing can be expected of any camera that is 40 or so years old.
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.