Well, for one, the off-road transportation is referring to the fact that it is modified for Off-road, should probably read "off-road suspension" like other vehicles. So you don't have to worry about that part of the cost. Its not one of the swapped out features. Since the workshop isn't specified, I (as a GM) would probably require the character to buy the necessary "Shop" to have the necessary parts to swap that in. Every possible shop isn't likely included in the price of the truck. And yeah, a Standard medium drone rack seems appropriate.
If your GM is going to give you a "used" price and make it not come with the interchangeable beds, I would probably go with something like 16,000 as the base cost. Replacing the missing hot-swaps would be somewhere in the ball-park of 500-2500 each.
Food Truck/Sleeper Command Cabin are similar to the concept of the Amenities; Food Truck presumably allows you to use the Armadillo as a food stand, making food and whatnot. And the Sleeper cabin would be a place to rest and sleep.
Transport is probably similar to the Increased Seating modification (and is probably why it has a variable Seats rating).
Armor, Cargo Pod, and Workshop (for the reason I mentioned above) are likely just different ways of arranging boxes/shelving to make it suited to transporting certain types of equipment. So these would be the lower cost ones.
I've always imagined the Armadillo as a truck like this:
where the part you put in the bed of the truck is swappable.