yes M13 is the best option if you can spent some extra money for your project and please try buy a complete set (Enginre, Gearbox, diffs) then this machine will be reliable companion for your trips.
I had owned a couple Suzuki long chassis and in the last one initially installed 3s-fe with hilux gearbox it was a powerful engine but fuel thirsty then i changed the engine to a G13BB with its original gearbox and diffs out of a 1999 Jimny wide. it performed superb in deserts and other terrain and excellent fuel economy.
and yes i have tried diesel in my previous vehicles but now i will not recommend you to opt for it as the body of suzuki is not capable to tolerate the vibration of a diesel engine and it starts generating alot of body noises.
regards.