Unimog Club UK

U900's are good, I have one. 

As for your wish list I'd be happy to buy one with everything except the front linkage as you could always make a tractor front linkage to fit, which would give you tight folding arms when you don't have anything on the front, they also have quick release balls on them so it's easy to put implements on, I converted my rear linkage to have quick release and their a godsen.

I have seen reverse drive front Pto boxes on ebay now and again so their not rare if you need one for your mower.

As well as ebay look in the farming papers, some dealers sell them without knowing their true price, a combine dealer near me had a 416 with a jetter on the back in his yard for ages at 3K but just advertised in the farmers weekly.

Mowing is good, but check out if it's ok using red diesel if you get asked to mow public land, seem to remember you should be on white as it's a tractor.