Nice truck!
Are those fold down steps on the outside of the rear box? That would be cool.
The rear should hopefully already have been mounted with Merc's three point flexing torsion free design so you could just build up from that. Redesigning, rebuilding and lowering is a lot of work and cost for a small amount of gain IMHO, and it will still be a big truck that will obviously still be too big for some places.
Too much weight too far back and the rear axle acts as a pivot unloading the front and eventually overloading the back. Keep it all as light as possible!
Add the overdrive for about £5K GBP plus fitting if it hasn't already (for reducing the gap between 7th and 8th which annoyed me and was a bit big, not to increase cruising speed), an exhaust brake and ideally working gears. New 14.00R20 or 395 85R20 Michelin XZLs if getting towards ten years old (they're date stamped), or just for peace of mind. Fire trucks tyres may get a lot of abuse!
When I first bought our Mog camper in about 2008 from Atkinson Vos in the UK I was concerned about its height at 3.9m. Someone with travel experience in Africa said you roughly spend one third of your time travelling, one third waiting, and one third living in the box. So too big and you compromise the Mog while driving, too small and you drive each other nuts while living. Our taller than most height was because it had the double bed over the cab which with a good map (to avoid low bridges etc) was almost never a problem. When we did get on each others nerves one of us could escape to the "bedroom", so that extra truck height meant we could stay sane more often than if it was a more confined box. The 3.5m max comes from
http://www.silkroute.org.uk/equipment/choosevan.htm but it's that one man's opinion from his experience. Someone on this site said the one bridge in an Ex Soviet country he came across at 3.5m they allowed him to drive across the train line instead.
Draw out the minimum size you want the bathroom, kitchen worktop, dining table (2,3 or 4 seats?), fixed or convertible or raising beds, and see how small a box you can squeeze it into. Look on Unicat, Action Mobil and Blissmobil's websites for what they think works, and the first two at least have been doing it for a long time. Or this one for a family of five but aiming to live mostly outside. Quite compromised I would think but worked for them for a long time;
http://opensens.com/photounimog.htm
http://opensens.com/unimog.htm
We sold our Mog when we went from two to three and a dog. Our new non-Mog camper box length is almost as small as one single, one double bed and a dinette (all fixed) I think would fit into and ended up 5.8m long.
Because of the torsion free mounting plus clearance to allow for the twist plus long travel coils plus big tyres a Mog will never be a low profile camper without an elevating roof. So I wouldn't get too hung up on the height other than with the same as length, weight, width, extras, battery, fuel and water capacity- as big as needed as small as possible. Which should definitely end up within Mercedes guidelines (probably
)