Unimog Club UK

Hi and welcome,
that's a fair crane you would like, something in the region of a 66tm with a dead weight of about 6 ton before sub-frame,  might be a bit much for a U900
The biggest problem I can foresee with a large crane on a unimog is the spread you would need on the legs due to a the base vehicle being relatively light, to achieve the stability required for the loler test, possible with double extension legs in each corner.
Its always the way, you see some thing that suits you for sale, and its the other end of the country.