U1700 as MR LR jnr said go the roof mounted option , I have a td5 wagon, and it's air con is not the best in summer, the only issues could be long roof rack/platform fitting and clearance heights. There was a bloke on here who fitted a roof mounted unit to a red early defender or county, sorry can't remember his name, maybe someone else does?
Anyway welcome to the forum.
It might also be worthwhile looking into a turbo inter cooler system for the Isuzu as there will be lots doing it maybe someone could start an options sticky for turbos on Isuzu going from standard factory to stupid.

