i was previously considering a Defender but now i plan to keep my current car and buy an RR Classic for green laning and off roading only, obviously i will need to drive it on the road occasionly but i want to kepp that to a minimum, i dont want to spend alot on one as it seems a bit of a waste to if its going to get scratched and dented off road so i will be looking for a late 80s Vogue i guess, my main question are what are the things to really look out for on these that are MOT failures or generally not good for the car to have?,is a 3.5 or a 3.9 better?.
also my only concern with one of these was the engine getting wet are there snorkel kits and engine mod kits for the plugs easily available?

main thing to check is rust on the chassis

3.9 would be better than 3.5

water proof kits for dizzy are on ebay or use a rubber glove and cable ties

snorkels are available but expensive, it'll be easy enough to make your own

rust is your main enemy here, the chassis is a biggy, plus theres a few areas prone to the rot, upper and lower tailgates, arches(all), sills, seatbelt mount points (mainly the rear where it goes into the arch), footwells, a pillars(inside) make sure you look under the trim. thats about it, and one other thing, the classics were bolted together with pig iron nuts and bolts, so when you snap em and they will snap, replace them with decent quality stuff, stainless if you can.