Unless you are dealing with a claimed low mileage survivor, mileage is irrelevant. Anything that has been "restored", restomoded, just plane modified, or built up. At this point it is all about condition. As mentioned, a poorly rebuilt engine is quickly worse than a regular somewhat worn out high mileage engine.
I've never owned a Bronco that I knew the mileage of. The odometer had a number on it. I'm sure that they rolled over at least once, probably a couple of times. Nothing ever had an original engine either. First one had an engine 3 years older than the Bronco that was from a car, second was engineless. I've helped several people remove an engine that was not original (at least the second engine) and helpd put in yet another engine (at least the third engine).
Now a bad body, that is very difficult to take care of. There is the throw a lot of work/money at it approach. I'm more scared of a bad body than bad driveline.