Scott,
I just happen to come across this post so I'm a little late...
The suspension on the P38 is quite nice. The rear arms are composite (I think) so they actually help the suspension cycle quite well. The air suspension has it's quirks and coils would prolly be better and cheaper for the long haul. Arnott industries carry replacement air bags; a much better alternative than the LR oem bags. However, I hear a lot about the BECM which controls the engine and all sorts of other devices. I think having one of these as a spare is the ticket. This vehicle is very hungry electrically so the alternator gets abused regularly. I think it's a Magento Marrelli or something like that and very expensive to replace. But, it should be cheaper to have it rebuilt I'd venture to guess.
The BW T case is quite robust I'd imagine. My old RRC had a BW Tcase and I replaced the chain and bearings around 120k miles so I expect the same rule applies to a P38. The viscous coupling performed fine in Moab, CO, CA and many other places. My BW Tcase used ATF instead of gear oil. I'm not sure what the P38 uses in the Tcase.
Sneak over to the P38 forum on discoweb to get a better sense of the common issues.
Interior appointments are very nice as are the seating positions; that was one of my favorite attributes in my RRC.
Be sure to get the 4.6 HSE; it's got balls and then some!
As with any LRs, you better be a comfortable field mechanic if you want to venture far from the beaten path!
Cheers.