[1leglance]

First and foremost you might want to do a bit more research because I have traveled for years through Central & South America and your reasons for needing an amoured rig is way off the mark....if you do a bit of searching you will read of unsupported bikes, motorcycles, VW vans and every other type of ride making this trek without any major problems.....
maybe a bit of hassle with the police at times but nothing outside the "norm" for that are...
Of course once you realize this then I would say you should look at outfitting a rig in either Canada or Mexico depending on where you are located. The reason for this is that you can many more options for large body, diesel powered Cruisers that would suit a long term long range expedition. Troop Carrier & Ambulance models as mentioned are great options...absolutely diesel and manual trans.
After that you are looking at how you want to equip it...camping vs hotels, bulk storage, keep the wgt low, stay with quality parts so it only cost once...all that sort of thing
another forum I would recommend would be expeditionportal.com
also you didn't mention timeframe of your build, budget range and if you plan to camp/hotel (this is important if you have to give up interior space to water/cooking/food or just all bulk storage)
welcome and keep us posted on the project