I have a hankering to play with m42.
I like some of the older Pentaxes and wonder how an M42 lens works (with adapter) on a K mount body. what features would be lost?
Any recommendation for a smooth M42 body?
Dave, am I guessing correctly that your older Pentaxes are K-mount bodies? With an M42 lens on a K body, you lose the auto diaphragm control, so instead you treat the lens as a "pre-set" and use the lens's auto-manual switch to open for focus and close for exposure. Not too bad, but subject to error. If the body has AE, then the aperture-preferred method will still work with the M42 lens, but of course stopped-down. AF confirmation will work too.
Be careful in shopping for the adapter needed to fit the screw mount lens to the K body... The film-plane register distance is identical for both mounts, so the M42 adapter must fit flush with the K mounting flange. Some don't, and have a thin flange of their own that will prevent infinity focus. The correct Pentax-style adapter is a bit fiddly to remove, as there's a spring-latch to move with a small tool or fingernail.
As to M42 bodies, this is a more convenient way to go, as you avoid the adapter and gain auto diaphragm. Spotmatics are the "classic" and plentiful choice, and use stop-down metering... as you turn on the meter with the sliding switch on the left side of the mirror housing, this automatically stops the lens down to the set aperture so you can check both exposure and depth of field. Models prior to the Spotmatic are excellent and compact, and have no built-in meter.
The last of the Spotmatics were the open-aperture models ES, ESII, and F. Backwards compatible with all previous lenses for stop-down metering, for open-aperture metering they require the SMC or Super Multi-Coated Takumar lenses having the extra aperture-keying tab. These lenses work fully on previous bodies, too, though without the meter coupling of course.
I have an ES, bought new, and an ESII, both featuring AE with electronic shutter control. The Spotmatic F is a manual exposure camera, and essentially a screw-mount version of the earliest K-mount bodies that followed, like the KM and KX (of which the popular K1000 was a stripped "price leader" variant).
Any of these 30+ year old bodies will need service to assure proper lubrication, cleanliness and adjustment. Just count it as part of the purchase cost, unless the seller has recent service documentation. I've been very satisfied with service from:
Hendrickson Camera Repair
272 Shoffner Rd
Sharps Chapel, TN 37866-2319
http://www.pentaxs.com/
I like Pentaxes, and have been using them since the mid-1960s... I hope you'll find a setup you enjoy!