Don't know abut the classic but the P38A will drop all the way to the bumpstops to crawl under things. The difference in height between motorway and extended ride height on the P38A may well be around 4" so with Access mode (bump stops) it'll blast that. I think the late classic system was very similar. If you get the Arnott Industry gen 3 bags then you get extended travel although obviously you'll need longer arms on height sensors and dampers.
Why not just get a P38? It won't rust as much, cheaper to buy, nicer to drive and with the money you'd make on the defender you'd have enough money to fit airlockers and 5 years of spare parts.