Hi and welcome Mick,
Glad to see that someone else has a Landy or two.
As for a Unimog, when you finally get one you won't know what you did without it. I have a U900 406 ( have a look at my introduce myself for full pics and desc.) and it's fantastic for my Arb / ag work, great for everything, and a joy to drive, most memorable moment was coming through the middle of Burton on Trent and whilst waiting at the roundabout fully loaded up with trailer and chipper, a person going around the island in a car slowed down, stared at me and lip reading he said 'I want one..' made me grin for the rest of the day as I have one!
The newer you go the bigger they are so i'm stopping with mine as a friend off here Nick, came around in a 1700 ( he's flogging a 1500 at the mo) and it was a tight squeeze around the house with mirrors folded in, where as my truck flies around without folding the mirrors in, only problem I have is how to fit two kids in it..
As for PTO HP yes the U900 has a powerful engine but the max Merc stated PTO power output is 45HP, which is around a 8 - 10" chipper so your right to look for a bigger one if your after 100hp.
Best time to look for me is allways from now until Jan on ebay as stuff comes up cheaper and no one has any money to buy.
Happy hunting, Ed