Seems like you are buying from someone who knows these trucks very well. Seeing that you plan on hauling weight and bringing two little ones, I suggest that you question the brake system, above all. The LDS upgrade is very desirable (especially if the air cleaner has been switched to one that can handle the increased CFM...SS members that did this particular swap have opinions on that) but you want to stop, too.
The hubs should all have been gone through to ensure good brake shoes, good brake cylinders, replaced flex brake hoses and steel lines checked for rust. Bearings checked and re-packed while in there.
Airpack and master cylinder original or replaced? Hydraulic brake light switch (they tend to degrade and break over time, causing brake fluid leak) or upgraded air pressure switch?
Consider, when/if you get the truck, to do the remote brake fluid reservoir upgrade right away. Cheap safety.
The rest is common sense; drag links tight, king pins tight, no wobbles or excessive looseness. If buying from a re-seller or dealer, any form of axle leak anywhere is unacceptable (unless you are buying a project truck) and while all Deuces mark their territory, excessive oil or tranny/transfer/PTO lube leaks are also a no no for something you intend to drive more than fix up from the get go.
Tires are another consideration; the stock 9.00-20 military tires are often of undefined age and need to be checked for cracks and dry rot on all sides.
The stock tires are fine if you do not drive much or are on unpaved surfaces often, but personally I do not like them for longer distances on hard surfaces and always cursed them when driving in the rain with an unloaded truck.
Lack of heater is - my opinion - overrated. The total lack of insulation of the firewall lets more heat through than you would imagine. Unless you are driving in the deep winter, not an issue. Also, heater can be easily added (take-offs available). Bows for the rear can be home made (purchase the metal bow corners) or purchased. Many members here probably have spares.
There is a good sticky here on what to check when you pick up a new-to-you truck.
The price depends upon the condition, options and how well the truck was gone through and repaired by the re-seller before you purchase it. For 5500 I would not accept a project truck.
My two cents - batteries not included. Your mileage may vary.
. I know that they are BEAST!