if its got more than 6 months MOT and its under a grand then you are laughing even if it does turn out to be knackered because you can nearly make that back selling it in spares when MOT time comes round. But i wouldnt say that a disco is any worse for rot than a fourtrack of the same age.
A TDi discovery will have similar performance to the fourtrak, similar MPG etc. spare parts a lot cheaper though. A discovery will be as good as or maybe better off road than a fourtrak, maybe ith a bit worse departure angle though. IIRC you cant tow 3.5t with a fourtrak but you can with a disco.