I don’t have any specific advice about buying an actual rental, but the best advice I ever got was that if you are looking for a solid, reliable car look at what cars the bulk of the fleet of a national rental place carries in the class you want to buy.
they typically don’t futz around with cars with issues and they have a lot more experience with them than a casual buyer would.
following that advice I ended up with the best car I ever owned–a geo prism. it was a car that would.not.die.