While I can't give much specific information I'd definatly agree with your assessment - M42 lenses are the cheapest game in town.
My dad gave me his old Petri M42, with a basic lens set. Pretty basic, but in good condition. The 135mm f2.8 has been grabbed by my wife, and with a $10 adapter is making good images on her Eos 5, when she's working with available light, and the 55mm isn't suitable. It's not the greatest lens, but as the Canon equivalent is £500, we'll get by with it.
I simlarly added a 400mm for £25, and a 24mm f2.5 for £20. A Teleconverter was £5. Any one of these would be in the £500-£1000+ range in modern glass. OF course modern lenses would totally blow away any of these older lenses, but that's pretty irrelevant if I can't afford them.
It's worth getting a camera just to TRY OUT these lenses. For a few hundred pounds you can have every lens going, and find out how that length works for you.
One sugguestion - if you get an adapter ring these lenses work fine on modern cameras in aperture priority mode. However theres no way for the camera to tell the lens it's taking the picture. This is fine for the older "Manual" lenses where the aperture is set, but newer M42 lenses are "Auto" - there's a pin that the camera pushes to stop down the iris. If you use Auto lenses with an adapter theres no way to stop them down (not always a problem, as we usually reach for these lenses when the light is bad). Ideally you want Manual/Auto lenses, as these have a switch - leave them in auto on an M42 camera, or manual mode with an adaptor.