NCV3 - Either you have factory support, or you can fluently modify mercedes module coding. Not really much in the middle, unless you want to toss some solid axles (and huge lift) under the van. Even then you are going to have to do some programming wizardry or at least some CANbus frame modifications in order to use low range.
The aftermarket conversions from europe are a good option (not cheap). If you could source a front 4x4 subframe, it would be possible to install a generic divorced Tcase, but you would still need to work out a low range fix. The other issue is that the 4x4 uses abs/traction control in lieu of locking diffs. The front driveline may not last long with a locked center diff.
T1N - Definitely more reasonable. But only if you can get the factory parts (never sold in the USA) and still not super common in europe. There were several variants, but one variant has 4hi/4lo with no center diff, locking rear diff was available. The front diff is about as strong as the rear (unlike the NCV3). Electronic-vacuum actuated, 5 speed manual, divorced Tcase. The front diff input flange is too large to fit the NAG1 trans without some fab work (not major).
Otherwise you will have to toss some solid axles under it, and figure out a way to make the auto trans work in low range (coding required).
Vancompass did a prototype where they used the front diff and integral transfer case from an Mercedes ML SUV. The gear ratio and T-case were mostly compatible, but they ran up against the low range TCM issues. There design used a bunch of custom work to bolt up the ML subframe. This setup was full time AWD with a 50/50 torque split.