With the accident damage waiver you shouldn't be stung for anything. A clean up of litter, mud and spills is just courtesy but not everyone does. I have collected cars that still had kids toys in the back and sandwiches in the glovebox. Trucks, however, have always been spotless, even the old tired ones.
With one car I had to return the contents of the boot to the previous user.
However, the driver would still be liable for any defects that would make the vehicle illegal.
I usually do a lights and tyre check and then cross compare the documents as a minimum.
I'll also try to check any electric windows, sunroof, central locking (works on all doors), just so that I can be sure the vehicle can be made secure when I need to park.
The hire company will have either a group recovery policy with the AA or RAC but some commercial vehicle hire may have their own, or local, garage service. You can check the limitations of this before you hire.
Some companies will have a 'return to base' recovery so, for example, your weekend in Scotland could be made pointless if you were nearly there and then recovered back to base for a replacement vehicle.
I was doing a house move from London to Salford when the truck broke down. The recovery man was instructed to take the truck back to the hire depot to tranship the load to a replacement truck. I convinced him to take me to my destination instead, as it was on the way, and then help me unload first before going to the depot for a replacement.