A few of the bigger issues and things you need to check before buying...Please everyone feel free to add to this list of basics for shopping. This is just a starter list.
Overall condition (rust, quality of maintenance, total mileage/hours, body/paint, etc., look underneath!)
Engine (make/model/mileage, leaks, diesel over gas best for just about all, Cummins are great...all mechanical preferred...turbo or non-turbo?...intercooled or not?)
Tranny ( avoid old AT 543's if at all possible (being phased out) but Allison's after that are excellent, the more gears the better...if a "standard", how is clutch?)
Gearing (many skoolies won't go over 50mph unless re-geared, others are geared for highway...need to know what ratio)
GVAW (actual total weight rating)
Axle ratings (front & rear)
Tires (check date as well as tread & condition-beyond 5 years not allowed in some states)
And, anytime you can acquire the units maintenance logs you are WAY ahead of the game.