I also wouldn't do anything beyond making sure it's in good mechanical/towing condition and clean. A good cleaning inside, a wash and a wax, and maybe a neat coat of white primer on the inside (if it's unpainted) to brighten it up would have more bang for the buck than upgrading gizmos. Making sure the required lights and brakes are properly functioning is also rather important—failure to do so, if not very clearly and explicitly disclaimed on the bill of sale and so forth, could conceivably lead to legal annoyances or worse.
