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I am not sure if I completely understand your question but the PTO is completely independant of the 3 point hitch. The 3 point hitch works off of hydraulics. Keep in mind that my hitch is actually at the rear. As for the lifting capacity of the hitch, I cant remeber and at the moment I can;t put my hands on my dang operator's manual!!! When I find it I will let you know.
As far as I know, all UNIMOGS have coil spring suspension. The number and stiffness of the coils is dependent upon what was ordered or installed. The coil springs give the truck a very smooth ride. Most people that I have given a ride to have been very impressed how smooth the truck is. In fact it is a touch bouncy without any weight when you jerk the brakes. I have been told that the trucks ride their best when loaded but I doubt that will every happen with my truck!
The truck you are looking at with the 6 tonne axle was a custom designed expedition vehicle. A fellow down in Washington State bought it. The axle was built by a company in Germany, I haven't seen any close ups, so I am not sure how it was all put together. The drive shafts are typically enclosed inside a torque tube and I would think that whoever modified the truck would have stuck to that principal.
As far as tranny's go there are several options. First there is the basic six speed with 2 reverse. Then you can throw in a torque converter on the same tranny, very neat stuff!
Then there is an 8 speed with 4 in reverse(I think). Then there is the 20 speed which seems to be an 8 speed with crawler and super crawler gears. If you look in the archives on the topica.com mogml list there is a very good and correct description of all the tranny's.
There is also a double clutch available that allows the operator to disengage the tranny and keep the PTO operating. Partly pressing down the clutch pedal disengages the transmission, pressing the pedal all the way down disengages the PTO.
There are also 2 PTO's, the first is a single speed, the second is a 2 speed.
I have a bunch of photos of my 416 and my old 404 at:
http://photos.yahoo.com/bc/samuelson_404
Also have pictures of my 2 other MB diesels there!