While we all love to talk about modifying vehicles to make them better for adventure travel, I'd like to suggest that you spend your time and money in a different way.
First, a year-long trip challenges any relationship. Make sure that both you and your wife are clear about your motivations and goals for the trip. You will have disagreements, and unexpected decisions will have to be made. Open and sincere communication is what keeps couples and larger groups together for such a lengthy trip. Talk about this with your wife, and give her the chance to express her goals, hopes, fears and anxieties.
Second, if you're planning to do extensive camping, make sure you take a couple of lengthy test camping trips in USA before you depart. If you can't get time away before the trip starts, then try camping in the driveway for 10 days without using the bathroom in the house. This will help identify issues before you go.
Third, if you're not already reasonably fluent in Spanish, take a course to learn the basics, and then seek out people you can converse with. (This shouldn't be a problem in San Antonio!) The folks I've met in Central and South America have been friendly and curious about me and my travels. They always warm up if they see you are making a sincere attempt to communicate with them.
Last, the vehicle you choose doesn't matter as much as we Expo-dwellers like to think it does. Unless you're planning to create a new route through uncharted wilderness, you'll be driving on roads that others use regularly. I'd recommend using a vehicle that's close to stock to increase the chance that you'll find mechanics who are familiar with the truck, and you'll be able to find parts. Take a couple of standard maintenance items like filters and fan belt. Expensive parts will have to be shipped to your location regardless of the make and model of the vehicle.
Don't worry about diesel vs gas. While diesel may be more available in truly out-of-the-way places, in 99% of South America, gasoline will be available, too. Sometimes you'll be buying fuel out of a nasty-looking drum, so have a siphon and a way to filter the fuel before it goes into the tank. Take a spare fuel can, too.
A USA-standard Toyota pickup will do fine as a platform for your trip. There's no need to spend money and time building a custom vehicle. It's better to save that money so you can extend the length of your trip!