Electrical engineering a specialty.
Rubs shoulders with Mechanical engineering:
It really is personal preference, and there are no hard and fast rules. Anything that a user did that was severe enough to significantly decrease shutter life you'd expect to show up in other areas as well. If it looks like it's been very roughly treated then that may affect internals as well, but otherwise it should last about as long regardless of mild external wear.
Switches do go faulty in time and amount of use can influence that. I've had two very high usage cameras which developed switch faults that were probably use related.
I'd try hard to get shutter counts from the cameras of interest.
Some earlier DSLRs had exceptionally low claimed shutter count limits.
Latest claimed lifetimes are often 100,000+ in middle range cameras, something like 300,000 for a D3.
Actual lifetimes can vary very widely with some figures of around 750,000 being reported (!) BUT cameras that have nominal shutter lifetimes of 150,000 clicks will fail at say 30,000. I've had two cameras that died with possible shutter failure from very heavy use (neither was one which had switch failure) but the death may not have related to shutter count in each case.
Grips and rubbery bits will wear with use and oxidise / perish / craze in heavy outdoor use, but that's probably covered by your cosmetic appearance ratings.
Electrolytic capacitors have a finite lifetime in use and high temperature storage with no power on shortens life more than when power is on. It's quite likely that a D90 uses no electrolytic capacitors.
If users use the USB port for photo download it can become worn and intermittent or worse. Any other plug in access, if used excessively or without care may become faulty. Apart from the USB port in some cases, most people do not use external ports very often.