In terms of seating, I've found the front seats good for two average sized adults and one kid / small adult. It works great with young kids. My son fits in the middle and it is actually nice to have him up front as opposed to by himself in the back.It was also great when he was in his kids seat. I've done long drives with three tall adults and that gets tiring fast though, especially if you have to shift a lot since the middle pax knees will get in the way. Definitely not ideal for three average to large adult males unless it is just for short trips. I've found the non air seats to be fine, even on long drives.I've done lots of long hauls with the stock army seats and I actually find them to be pretty comfortable, but I come from old school German style upright seating ergonomics.
It is not worth removing high speed gears for a Claas. The claas gives more gearing options but not really a better final drive ratio and the high speed gears create less wear and tear and are one less thing to have issues with. If you have the high speed gears, it is well worth keeping them. Adding a Claas to high speed gears will really help 100-110 kph cruising though, especially with a turbo OM366A. As others have mentioned, turbo is a big bonus with fast axles and/or overdrive.
In terms of boxes, there are a lot of options. Do you want something removable or permanently built on? What level of creature comforts do you want? I've got a turn key ambulance that is basically a stripped camper. It is an awesome size, especially if you want to do tight trails and keep the dimensions tight. Down side is you can't stand up inside and they are pretty basic. Blissmobil is on the other end of the spectrum. Really comfortable with all the bells and whistles and with standing room but very tall and heavy. If you don't need a removable back, there are other options that use the space more efficiently. There are quite a few places in Europe that will do anything from a shell to a full out finished box designed to be permanently mounted on the vehicle. There are also Zeppelin shelters that can be easily converted into campers, but again if you don't need to take the box off, it does not make the best use of the space in the back. Zeppelins are great though for a weekend camper for a work/play U1300L or 1200L It really comes down to budget and priorities. However, weight becomes an issue much sooner than most people expect, especially if you are going to be carrying extra water, fuel, tires etc. A U1300L can do pretty good off road at the GVW but if the weight is not distributed well it can really effect the capability of a U1300L. The height of a box also should be considered as much as the rear departure angle. I've seen more expedition vehicles limited / damaged due to height than break over angles.
