After underside rust, cosmetics, the dozen bushings and rubber bits on the suspension and health of the engine and transmission? (since those are the same for any unibody car with CIS), the 6.9 centric thing is the hydraulic suspension system.
Basic observations...
Does the car sit right when it's been unused for a day? (take a picture with your mobile)
If not, does it fix itself when you start it?
There's a lever on the dash to control ride height. Does it work right? (check the owner's manual)
When riding at the correct height, how does it feel when you drive it?
Do pot holes, speed bumps, left & right turns, quick braking, quick starts, braking on a turn, all feel right?
Shut it down. Does the right height stay put for a while?
-CTH