Give the V8 a good rev when hot, listen for any knocks..
Check the oil as said, these engines need to be kept serviced, the cooling system too, make sure it has clean coolant in there!
Give it a good hard test drive and makes sure it doesn't puke its guts up or weep some water out the expansion tank..
Rust is the killer, the front floor just behind the front wheels needs a good looking at mine was crispy there, quite a few others i've seen
Chassis the rear especially, the chassis legs next to the petrol tank can de-laminate.
All in all i love the D2, a great looking 4x4 and a great performer!
Just find a good un and you'll be golden, a nicer car all round over it's replacements IMO