Given that R2R is not big except in some recording studios, I would be looking for a Studer machine if you want professional quality mechanics and electronics with balanced inputs and outputs. Revox was the semi-pro line but not nearly the same quality as Studer.
You could also be on the lookout for an Ampex ATR-100 or an Otari MX-5050 or a Nagra IV.
https://reverb.com/item/4118913-otar...nally-restored
Watch out for heads that are too worn, they can only be lapped so many times before the gap is exposed. Many parts may no longer be available, pinch rollers etc.
Getting machines working and calibrated is tough, not many techs know how to fully align a machine.