see
What to look for when buying a Land Rover Discovery
However, no purchase check list beats an hour of a good mechanic's inspection.
Common problems are oil pump cracked and failing (mechanical PSI test a good idea), front drive shaft does not have grease nipples, causes shaft to fail and spear a hole in transmission, head gaskets, overheating (engine should be able to drive plus idle 20 minutes without needle going above 9:00 on gauge). Oil leaks expected.
Most common problem is that once in your driveway, it becomes your new master.
As far as commercial, they did make some without rear windows, but seldom see those in US where this forum is based. If shopping in the US,
www.statewidelist.com will seach all of craigslist for you, in a certain state, and output by metro area.
Usually a good idea to look at several. But deals do pop up, best in recent times was a gal who bought her first car, a D2, for $2000, 117 miles, garaged since purchased by eccentric owner.
As far as purchasing a commercial fleet used Rover, would not think that would be a good idea in the US. These vehicles take too much maintenance for fleet use. So I would suspect that the vehicle would have suffered. Of course, that is also true of the "soccer mom" truck, driven 5 miles a day, oil changed once a year, etc.