Sort of like checking out a used motorcycle . . . has it been dropped, how loose are the controls, does it show wear in any parts. If it looks fairly new, and well cared for, then it might be fine. Another issue might be focus or control stiffness, if it has sat a while, though easy enough to remedy that.
I needed to go back to a 4x5 for some work, after selling my Toyo years ago. While I checked in used cameras, I personally felt better going with a new camera. Rather than tell you what I bought, my suggestion to why to buy new is to consider from whom, or what business, you are buying your camera. If you can get a return policy or agreement, then I don't see a problem getting a well cared for used camera. My buying new was my own peace of mind decision.
Decide what you want to photograph, and which lenses you might want to use. A separate issue is what types of film you want to use, or if you would even consider a roll back, or a Polaroid pack film back. It could be nice to have the versatility, but if you don't anticipate using it, then a spring back is fine. Wide lenses can go on recessed boards, while some extension boards could allow slightly longer lenses to be used. Ideally, you don't want to outgrow your camera, though you can always buy something else in the future.
Ciao!
Gordon