Springs rarely fail, on some brands they might rust through, but usually due to a place water can trap and cause this. I suspect springs on the Land Rover will be fine for another 40 years...I've had the medium springs on for about 2 years now , when they start to fail maybe look for a more softer spring and keep the 18's
Sounds like a good idea , would you go 16" alloy or modular
They said they have 265/65R18's, that is not a standard tyre. We aren't talking rubber band thin side walls here.16s much nicer ride than 18s, especially on todays roads!
Might lose some steering lock with the wider tyres.
Tyre pressure play a big part, you cannot go to far wrong with 30psi as a starting point.