Two spare tires. I guess it depends on the scope and duration of the trip. I have done multi-thousand mile trips that included hundreds of miles of hard off-road driving with no spare tire. I always have OBA, plug kit, a way to break down the tire, patch kit, and a last ditch tube. I have been using my old Willys flat fender for the last few years. I did finally pick up a spare tire and designed a storage solution, but it isn't my favorite. I don't have any problem breaking down tires myself and repairing them. I guess it just boils down to comfort level.
Rotating mass. I don't think it is THAT big of a deal for off road use. We aren't trying to set 0-60 records. I do think braking performance is often overlooked, but I see that be an issue because of the total vehicle weight as much as tire size.
Leverage. On a steering axle you can't really work around this. Since the entire knuckle has to move, you are generally always isolated to the bearing/balljoint system at the kingpin axis. The other interesting part with bolt-on portal boxes would be the additional offset, outboard, in combination with the leverage from the tire being effectively taller. Most factory portal axles like the Volvo C303 or unimog axles used very high backspacing to keep scrub radius ( and knuckle leverage ) in check.
Axle Parts. Axle parts. While the axle parts remain standard to some degree, most of the stress is going to be on the custom portal boxes. If they are built right it shouldn't be an issue. Custom is custom however. I would hope that the brake, wheel bearings, etc would be standard parts.
The Tibus axles are a very neat 4 gear design that keeps everything spinning the right direction. Most OEM portal axles use a 2 gear design and basically 'flip' the rotation of the ring and pinion relative the vehicle direction....Volvo, Unimog, Hummer, etc.
