You might want to make a list of all the features you want (and even everything you want to examine) and check them off as you look at the camper. This will also get you to focus your attention on many different areas.
Before we bought our Northern Lite, we rented a NL Sportsman (on a new Ford) from CanaDream. Noticing what we liked and what we didn't, we decided that we wanted a NL 10-2 CD (which was not what we previously thought we wanted). Moreover, we discovered that some of the used NLs had more of what we wanted than did the new ones.
The 10-2 CD has enough headroom for us to sit up in bed and more headroom over the sink. Plus, it has cabinets over the dinette. The Sportsman did not have these advantages.
Plus, turn everything on (not at the same time) to see if it works. For example, one of our Happijac motors did not work the first time we unloaded the camper. The seller subtracted the cost of replacing a burned-out Fantastic Fan motor (which he brought to our attention).
And ask the seller to explain how to operate everything, especially things you're a bit fuzzy about.
We love our Northern Lite. Good luck with your looking.