check ALL of the features and options. Don't assume that the vehicle has any feature that you've seen in review videos or read online, it's probably an option, and your specific vehicle may not have it.
Make sure you check the functionality of the HVAC on both sides of the car, don't assume that becasue you're getting hot/cold air from the drivers side that you're also getting it from the passenger side.
check to make sure all of the little plastic bits are secure. The covers on either side of the rear center brake light seem to be touchy, in particular.
Find out when the battery was last replaced. You're RIGHT at what could be the end of life for the battery. If your battery can't hold a high enough charge there are several features on the vehicle that simply won't work anymore, but the voltage cutoff at which the truck won't actually start or will throw a warning is lower than the cutoff for specific features, like the tailgate life and the auto start/stop
These are just my observations, having purchased a 2016 HSE just 3 weeks ago.
I love it to death, but it has required some "shake down" and a few trips back to the dealership to tidy up little issues that weren't noticed before purchase, fortunately the dealer has been great.