One more thing - can all backs go on all bodies? All lenses too? Lets say I got a cheaper Pro body from KEH, but in a few years wanted a nicer one. Would all the backs I have still work, just without the double exposure interlock?

Also, if you were to own 1 focal length lens, what would it be? 90mm is "normal" for MF correct? I'm planning to give some portraiture a go if that influences