If 34s on 18s would fit even with air bags I could potentially get past having the bags but I still like the long-term reliability and simplicity of coils better. Height sensors go and air bags leak- perhaps the Defender's air system will be worlds better, but we won't know that until they've been out for years.
I'm just not sure you will be able to fit 18s- the calipers on the pictures I've seen don't appear to show much room and they are on 20" rims. If you can't put on larger tires- or at least if it's incredible difficult to- like with the LR4- you lose the pliability, traction, and resiliency benefits that come from airing a tire down off-road.
What are everyone's thoughts on the likelihood that the radiators are going to sit in front of the wheels- that seems like a vulnerability?