Unimog Club UK

What sort of speed can it comfortably do with a load of logs on there? It's annoying that these things are so rare - I've never actually seen one working! Driving around an off roading site even with no payload and at low speeds doesn't give much away as far as every day use is concerned.

If I manage to get one I'm hoping I can get some paddock mowing work with it. Stick a 6' topper on the front and I recon it would get a fair bit done!

Also, what sort of fuel consumption do you expect when running the splitter?

its not very standard, highly modified engine. 
It produces 155 bhp and a LOT of torque (yet to test), put it this way, it doesnt know what hills are!  hence the larger wheels, which give it a top speed of 60 mph, 56 comfortably. to get the tyres on comfortably we had to lift the cab by 2", were currently on the hunt for some high ratio axles.
normally a standard 406/416 will do 48/50 mph .

were going to be doing some tree work i the next few weeks hopefully, so i'll take some vids 
i wouldnt worry about everyday use, its faster and more versatile than your average tractor, and can do more than your average 4x4.
Regarding the topper, i recommend you go for a rear mount, early unimogs are designed to carry a lot of weight on the back, so the engine is far forward, which means that your better of putting implements on the back if you can, my dad does paddock mowing for a bit of pocket money and has exactly what you said, a 6' flail, but on the back. then we have the mulag flail mower for cutting hedges, and also getting right underneath hedges, together its a proper unit! 
if you stick it on the front, it will do a tidier job, because the wheels of the mog arent runnning the grass over first, but counter balance the weight by putting a weight in the back. 

with the splitter i have no idea about fuel consumption, but an average 406 will do around 14/18 mpg depending on how its driven.