If you are going to put a digital back on it some day, I'd go for any 4x5 with a standard Graflok style back. It has been the standard for decades now, and I don't imagine it changing any time soon. I don't know anything about digital, but all digital backs for 4x5 cameras must be compatible with the Graflok system. 4x5s are not really large cameras, so I wouldn't worry about that.
If it was not for the digital thing, I might suggest getting a medium format view camera or SLR with movements, such as the Fuji 6x8. That would give you movements without having to move up to sheet film.