It depends on how much extra fuel capacity you want. If you are talking only 10-15 gallons, I would go with Scepter military fuel cans on a rear bumper (or at the worst, a roof rack). If you are talking 20 gallons plus, you are looking a subtank. But then you have to relocate your spare tire. The dead nuts cheapest would be a couple of Scepter cans in the rig or thrown up top. About a $200 plus hardware. Otherwise, you can go into the 2k+ range with a subtank and all the plumbing.
Tires, go with something strong and not too tall. The tires Michelin makes for desert driving in Africa are ideal, but we can't get them here. Best bet would be a 31" to 33" all terrain. I went with Toyo M-55 in the 255/85R16 size...about 33" of 10 ply goodness. Nice benefit is they have a great footprint aired down...not so nice...20lb's more per tire than stock. I posted a couple of pics on the 'What have you done to your 100' thread...
Skidplates are six or half a dozen of the other. Aluminum skidplates are light but expensive and don't hold up well to dragging your 100 through rocks. But you really shouldn't be doing a ton of that driving down to South America. Steel skidplates are cheaper but weigh a lot. But they are super strong as well. I have steel skids from BIOR...overkill definitely. I may have to trade them out depending on how the 100 builds out and the final weight of the rig. Shooting for 7200 to 7500 pounds max.
Watch the weight of the truck...many of the fully built rigs on here, when loaded up for travel with people, gear, and gas, weigh over 8,000 pounds. That is significantly over the GVWR of the 100...they handle fine, but gas mileage goes down and the rate of parts breakage could go up.
