The interesting thing about the RV industry right now is that nearly every manufacturer uses the same few vendors to source the major components of the units they build. You look at my Jayco and you will find a Colman Mach air conditioner, an Atwood water heater, Lippert axles, etc. Look at Mike's Cougar or the KZ you are considering and guess what, you will find them there also. Of course there are differences in quality of materials and features that very across the price points, but when it comes down to it, they are all really slightly different versions of each other
So as mike said, when you look at this KZ, make sure that it has been well maintained, no signs of water leaks or damage, no soft floors, water stains around the windows, etc. Get up on the roof and inspect the Dicor around all of the vents and openings, make sure that the roof membrane is in good shape and if there are any patches, that they were done correctly with Eternabond or a similar product.
Make sure that you test each appliance and make sure they work in all modes. Make sure the fridge will work on both gas and electric, same with the water heater if it is dual mode. Use the water system, the pump etc. Run the furnace and make sure it cycles on and off correctly with the thermostat. If it has a slide, run it in and out a couple of times to make sure it operates correctly.
Hook it up to your truck and make sure that all of the lights work, test the brakes, etc. Don't assume that anything works until you test it for yourself.
Remember, there are thousands of used units for sale on the market every day. If you look at the KZ and like the layout, everything works as described, and you find no issues with it. Then by all means buy it if it is in your budget. If not, walk away and search for another one.