Unimog Club UK

ive already given my view on the wheel base options, its all down to what you require  and also on the pros and cons:

416:
more stable off road
larger load space
sometimes came with high ratio axles
better road handling

406:
more maneuverable
easier to store
look better 
sometimes came with high ratio hubs (but rare)

1150L 416:
longer than the 1100 416

Yep, the full body can still tip on the spider frame from a 406 on a 416 (they use the same spider frame) and yeah, no reason why you couldnt fit a tool box, its quite a common thing to do.

the 4 speed box gives the bonus of being able to reverse quite fast, they were used on quite a lot of rail road mogs, 4 speed splitters are worth a bit more than 6.
6 speed splitters give a broader range of gears, id recommend a 6 speed for you.
cascade can be fitted to either box, and is of course a bonus, hopefully this helps
http://www.unimogclubuk.com/tutorials/u800u900u1100-gear-levers/

sourcing pto shafts is easy, as you can go to any good prop shaft maker and get them made up, finding the end bearing blocks, and the center bearing block, is the hard bit. 900's and 600's share the same bearing blocks.

and yes, the 416 is a rarer machine than the 406, people seem to chop them up for the 406's