Seen those images, scary stuff. From what I gather though those aren't rust treated which will make a huge difference. Not that it should be needed, but still
Will a disco do 40mpg?
also, discos are about £5-8k more here for same age an milage
Depends on your price point. Newer 2.0SD4 is supposed to do 43mpg. My 2007 2.7 TDV6 does 30mpg consistently regardless of load. The newer you cab afford the better the consumption will be obvs.
They're supremely comfortable and pleasant to drive though.
The same Nissan Navaras that routinely fold in half behind the cab due to chassis rot?
Get a Discovery instead.