Great start, I'm in! I am sorry to hear about your father in law's condition. I just finished riding all the countries in South, Central and North America in May. This was spurred on by my bout with Prostrate Cancer five years ago and the realization that I would not live forever.
I found South America more enjoyable than Central. That maybe had something to do with the final rush through Central America to get home. But, if there is any doubt, I encourage you to make Argentina, especially the southern part ... it's the most beautiful. But my favorite country remains Colombia.
I think there were a couple pieces of advice that I found very helpful; 1) less stuff, more money and 2) be friendly but learn to ignore the ingrained US tendency to follow all the rules. Most border agents, police, speed signs and yellow lines are there to be ignored. :-)
Depending on where you go, you may also need insurance for your bike. Colombia, Peru and Argentina make a big deal out of that. And finding it can be a real pain. Some places you can buy international insurance that covers more than one country. One thing I realized though is that I could show most police a copy of my insurance card from the US and get away with it. If all else fails, "por favor, no habla espanol!" There are also some others that require insurance, that don't seem to be on your planned route, like Venezuela.
I had also purchased an International Drivers License but felt like I was getting a secret decoder ring from the back of a cereal box. $8 and a copy of your drivers license will get you one. But, it came in handy once or twice at border crossings.
El Salvador, Panama and Ecuador use US Currency as their currency, but most other countries will except dollars in places. But, I always found it was good to have a little local currency, so I stopped at an ATM just after entering the country or used the "cambio" (currency traders) you can always find at the border crossings. I had no issues with rip offs with either the ATM's or cambio, except in Honduras where a cambio convinced me I needed Lampira (Honduras currency) to enter El Salvador. He got me for 50 bucks.
Enough, enough ... I could go on but I will stop. Good luck on your journey man! If you are passing through Houston on your way south, drop a note, I would like to buy you a beer. We may not be Boulder, but we have some good handcrafted stuff here. We are a good one days ride to the Mexican border.