TeriAnn
To set a context for my reply, I began Jeep driving in a CJ5 all over West Africa in the early 1960s, with (unrealized) hopes of driving - unsupported - Morocco to East Africa (silly me!).
Now we are running a 2008 Rubi Unlimited. In my opinion, if one wants to run inimproved roads and trails, you can do 99% in a stock Rubi, and if you have 2 of your kids on board you aren't going to (and shouldn't do) the other 1%. So, #1: Advise them to go stock. This vehicle is REMARKABLY well-set up as is!
The back end - with the seats up - will easily hold: 15 gal extra gas in 3 metal Jerry cans secured to the stock tie downs; a PowerTank to refill tires and operate air tools; 5 gal of water in a Jerry can; stand-up tent for 6; two cots for the adults; sleeping bags; a cooler for two to three days of food; pots and a single-burner gas stove. At least, mine does! (When we use the back seat space, either up or down, all sorts of luxuries can be included!.)
I think a family that puts too much emphasis on technical changes to the vehicle is making a mistake: Ego has captured the real goal of the exploration.
If more room is really needed by your clients, advise them to buy a trailer.