I have canon 40D and use M42 and Pentax K mount lenses. I had 400D before and I find the upgrade to a larger viewfinder VERY helpful. I use chipped adapters so I get the focus dots and they really do work. Even better is that 40D is built to take different focus screens, so it is easy to change them (takes 10secs), and such operations can impact AF system, but you get a menu setting to compensate.

You might want to interchange them based on situation, as there is no free lunch. more precise means darker, because it diffuses the light more. I dont use katzeyes because they black out at lower apertures and they obstruct your view. Focus screens that improve your perception of focus are darker than normal , so you want the brightest viewfinder you can get.

Itemized benefits of a "proer camera":

1. Brighter is better! - easier to see focus on a bright image. especially with precision focus screens you need it brighter, because the screen makes it darker.

2. Bigger is better! - read on the DOF and perceivable focus as a function of viewing size. 100% crops almost always look horrible while the downsized images are nice - because small size hides all the mistakes, such as out of focus.

3. Interchangeable focus screens with matching menu setttings: adapt to the situation without screwing your af system.

4. not really about the camera, but focus screen aids are optimised for F2.8 or faster lenses. zooms arent usually that bright, and it might make it harder to judge focus! go Prime.

as a side note: vintage lenses are awesome, but back then they didnt make zooms really well. So it is better to go for the primes with good review on pentaxforums lens review section.

Also note that if you go too pro (full frame) you might damage your mirror, unless you adapt the lenses.