I agree to get what you like.
A '98 Tacoma is not old at all. I consider that to be quite new (I drive a 1985). It's impossible to foresee the future, but that's a very reliable, proven platform and chances are it will make the trip (and many more) without a hitch. The 3RZ is quite fuel efficient. You should be able to average in the low 20s for MPG, depending on how it's loaded. Chances are, a 3RZ Tacoma and a Vitara will deliver similar fuel economy.
It's true that you don't need a 4wd at all to complete the trip, but there are other places you can access with one (such as beaches, mountain trails, etc). Definitely take some time to learn 4wd driving and recovery techniques before you depart. There are books, videos and courses available. Local 4wd clubs can be a help. Driver skill usually makes more of a difference than the vehicle in terms of where you can go safely.
There are plenty of trip reports from others who have completed the journey--you may want to browse some of them. Here are two that used 1st-generation 4runners (both 1987s).
http://www.expeditionportal.com/for...hread-87-4runner-built-for-epic-PanAm-journey
http://www.expeditionportal.com/for...way-San-Francisco-to-Ushuaia-in-an-87-4Runner
Since you mention you've driven a Prius, I also drive one (a 2nd-generation liftback) and if I were to embark on a long journey, I might be more tempted to take it than the 4wd camper. It literally gets 3X the fuel economy, is drop-dead reliable, has nice cargo capacity and can be slept in. It even does pretty well on sandy two-tracks.
Definitely select carefully when making modifications. Don't feel the need to make the rig
LOOK like an expedition rig. What is an expedition rig? A rig that is used for expeditions. There's nothing in the definition that says it needs to be outfitted a certain way. Focus on maintenance items rather than modifications. Only change what really needs to be changed for reliable, functional operation.