talk to the people that have actually done this in the touring forum.

But the simple answer is that you can do it on anything.

Most vintage touring bikes wont take tires much larger than 32-35mm without a fender. I'd ideally rather have something wider, though I've toured a few thousand miles on 25mm tires. I'd have to air them up every day though to keep from getting pinch flats. Higher volume tires are nicer in that they lose air slower, or at least it seems that way.

So out of the suggestions so far I like a vintage MTB conversion, or a used LHT which isn't vintage.