I have the EOS 5/A2 and indeed it is the best optimised EOS save for the unfortunate programme dial
. I think I've read in the past of at least two Magnum photographers using one, when the camera was current. Mine was a gift from a friend who used it lightly from new, which is how I also used it from the moment it fell on my lap. After a few years of use the dial did brake. First it lost the detents (but still selected the intended programme) and after some time, perhaps a couple of years, it started freewheeling without selecting/changing programme. I most definitely used the lock button at all times and without exception (I was aware of the problem), so the problem did not occur out of misuse. I believe it's still possible to make a repair, but the cost is probably more than buying a replacement camera. It may still be worth it because it's a good camera, I just have not gone through with it yet. If you find one that had the repair already, then it probably has quite some time before the problem reoccurs. If it hasn't, it's really a coin toss.
I replaced the EOS 5 with the EOS 30v which, being a newer camera, still has a few years of life in it. It is not a better camera, it just has some later features while skipping some of the things that made the EOS 5 great, for example, the brilliant IR AF assist light. To get around the lacklustre low light performance of the EOS 30 I use an ST-E2 AF assist. I also really like the EOS 100/Elan, though I can't really recommend it, because of the sticky gloop that eventually kills the shutter. But it's a camera that works for me (when it does
) and it's really cheap as chips.
EOS cameras readily accept adapted lenses -- M42, R, F, C/Y and so on. Before the advent of mirrorless it was the obvious choice for adapted lenses. But you do have to use them in stop-down mode and, usually, with viewfinders optimised for AF rather than manual. (Not that much fun, as far as I am concerned.) I've never used an AF-confirm adapter so can't comment on those.