I've had a few full time Jeeps with QuadraTrac. The first a 1978 Gladiator with a 401.... no idea what the gas mileage was but it was expensive to run and I sold it. Then a 5.9 Grand,,, wonderful vehicle but the transmission eventually died and the $3500 bill made me say never again. I'm committed to clutches and part time 4WD now.
On the gas mileage vs 2WD vs 4WD.... My YJ did many 950 mile commutes Edmonton to Yellowknife. 2WD/4WD made no difference to the gas mileage. Some winter trips thru storms were 4WD from Edmonton to Yellowknife, most were 2WD but there was never an "oh wow 2WD gets better gas mileage". I stopped for gas in the same places and the fill was always the same.
I love the reliability of old fashioned mechanical shifting systems way more than some rotary switch on the dash. In the 1970s, I had a rear driveshaft explode... on a 1963 Chev 4x4.... pull the driveshaft and shift into 4WD,,,, now FrontWD and keep going. It took 5 minutes to get going again. Drop off the driveshaft for a rebuild and bolt it in a week later. I am definitely a fan of old school.
EDIT
Also a fan of buy whatever is popular, common where you overland. I've seen many cool but "odd" vehicles stranded waiting for parts cuz they "shipped" their vehicle overseas..... If you go halfway around the world make half the adventure shopping local for the most common wheeler they have and make the build part of the adventure.... even better if you learn a new language in the process.