I have two prime M42 (screw) lenses, which is a great, used <£1000 30mp+ DSLR I can use them on with inf' focus without damaging the mirror?

I am not aware of any M42 dSLRs - so you will need an adapter to mount it, so I guess that counts for real justification. Which camera are you planning to use and which lenses (brand, focal length). The only problem I had ever had in that regards was with an adapted (from M39 SLR mount) Mir Ib  on a Zenit 12 XP. But in general you should be good to go with a M42 to whatever you are using adapter. M42 is a SLR mount so the mirror was taken into consideration when building them. But without knowing what camera you to plan to use them lenses on it's hard to say.

With Nikon F-mount cameras you'll need an adapter with a correction lens so you can

a) mount it

b) reach infinity focus

this will degrade image quality.

With Sony A mount, Canon EOS or Pentax K cameras you can use glassless adapters that don't degrade IQ. You will need the respective adapters which are available cheaply to be able to mount the lens to your camera.

Just about any DSLR or mirrorless can take M42 lenses with an adapter - the one I'd worry about is Nikon, which needs a corrective element to maintain infinity focus.

It's easiest with Canon and Pentax DSLRs, as the adapter is a simple metal ring. which clips into the lens mount, but mirrorless adapters would be a short tube to increase the back focus.

There are no dSLRs made with the M42 mount. The closest you can get is a Pentax dSLR with a K-mount. Same lens index. Any Pentax dSLR will do. The M42 to K-mount adapter can get as low as $6. Get the one that's fully flushed when mounted for correct focus to infinity. The cheapest fully working Pentax dSLR I found in the secondhand market today is $60. It's a 6MP Pentax *ist D.

It is exactly for this same purpose that I maintain a digital Pentax K-r, so that I can continue using my box-full of M42 lenses. I even went further and swapped the K-r's focus screen to the ol' split-image microprism but that's optional. You can easily focus manually with the default focus screen of a Pentax dSLR because it's that bright.

Nikon, Pentax and Canon DSLR cameras can all use screw mount lenses with a simple lens mount adapter that would allow you to focus the lenses to infinity. These adapters have no linkages to the camera so you will need to use stop down metering, manual focus, and manually operated aperture controls. Therefore you do not need to worry about which brand to buy because all brands of DSLR cameras can accept screw mount lenses. 

If it's an M42 lens it was designed for a 35mm (ie full frame) camera, so the Canon 5D MkIII would be a good choice - but it's less than 30Mpx!  Do you *need* >30Mpx, because finding something within your budget just got a whole lot more difficult!  You might find a Pentax K1, but they're few and far between, even with the introduction of the MkII.  

Please be aware that some M42 lenses extend into the camera body more than usual - you need to make sure your lenses don't fall into that category.