Need a little more info.What brand and type of Engine is in the truck.Do have the maintenance records to be sure that maintenance was performed on schedule. I assueme it is an expediting truck so if you can find the company it was leased to,you should be able to find out how it performed and most important if it was involved in an accident.
Open the hood and if you see a yellow Cat 3126 be careful.Lots of stories on this forum about that engine needing major repairs at about 400k.There is always an exception as Dave KC can prove. Also what is the price they want to sell it at,what size sleeper and box is important.