Thoughts.... and deeds...
The Retina to M-42 adapter is expensive compared with the price of the lens and camera that it would fit. You can pick up a Retina IIIS for ~$100. The Rangefinder body is much simpler than the SLR. The top comes off easily and the viewfinder is easy to clean. The Compur shutter is easy to flood clean, just take off the lens.
The SLR's: The Retina Reflex-S, either the shutter goes bad or the prism goes bad. Find one with a working shutter and bad prism: buy a parts Minolta XG-1, XG-7, or XG-9. The prism is a perfect fit, and easy to replace. My Reflex-S was $50, the parts Minolta was ~$5.
Retina lenses are cheap as so many SLR's have died. The 35/2.8 Curtagon, 50/1.9 Xenon, 85/4 Tele-Arton, and 135/4 Xenar were ~$100 total. Cheap German Glass. Lots of Character.