based on my experience in mexico/central america---
speed is unlikely to be as important at reliability. I would think twice before adding a turbo.
7.50r16 tires are a good choice- easy to find spares if needed.
If needed for weight concerns, you can carry one spare tire unmounted.
Keep water and fuel low. I know you know that.
Rooftop tent is less water-proof than inside the vehicle, but it will be damn hot inside most nights. Provide for an awning of some sort off the tent and you'll have the best solution.
Cooking under an awning beats the heck out of inside the vehicle too. The folks at
www.newcruiser.ca had a great awning solution taken from the boating industry-- a C-chanel mounted to varoius areas of the vehicle into which you could slide a tarp. Basically- a lot like a sail. It worked great till they had it stolen (note: a grommet w/lock would be a good idea)
On the topic of stolen stuff: locked storage for little things is only useful if it is A) easy to access/open/close from driver & passenger position and B) fairly difficult to see when closed. Tuffy consoles are excellent in both regards. Locking glove compartments are also good. A custom designed overhead console would be awesome as well.
Something that many overlook but is super important: 12v Fans. one or two for the tent, one or two in the vehicle (you can use the same fans). Hella makes some awesome 2-speed turbo fans that are small and light and move a ton of air at milliamps of draw. We used ours every single night while travelling and usually would have been happy to have had a second fan.
Two light sources in the tent is also a nice idea so that one can read without the other getting grumpy.
I'l think of some more stuff later, i 'm sure. Sounds like a fun trip!