Touring - bike recomendations?

i am a college student that is getting back into riding. i am 23 and quit biking about 4yrs ago. my wife and i are planning a trip starting in Brownsville, Tx and ending in Panama. i no longer have a bike so i will be starting from scratch again. there will be a lot of road riding but also some really rough riding. there will be about 20 of us going, and for this reason we are taking a bus that will contain food and medical equipment along with repair equipment. my question is what type of bike sould i use i have thought abobut a mtb bike but the Trek techs recommended a 520 i am not sure this would be a sturdy enough bike for what i have in mind. thank you for any imput. lou

With all that off-road riding, sounds like you need a 26" MTB-wheeled TOURING bike. These are quite common in the UK (Thorn, Orbit) but the only resonably priced model in the US is by Bruce Gorden.
http://www.bgcycles.com/blt.html
BG bikes are ready to tour , unlike Treks which need extensive modification (lower gearing, servicable gear changers, stronger racks), so factor that in when you judge the price.

Compared to an MTB bike, the 26" tourer is designed specially to carry luggage with strong attatchment points, a lower centre of gravity, and steering geometry for loaded performance. The BG racks are just about the best you can find.
Drop bars can give the same cruising position as MTB flat handlebars, but offer more alternatives than bar-ends can.
The bar-end gear shifters are easy to use and very reliable. You can keep your gear system going even if you lose indexing.

You aren't going to find such a specialised machine at your local Trek boutique. Having been to Mexico, I can assure you that you will be seeing dirt, broken pavement, etc, and that you need a bike that has Schrader valves! Look for a specially designed 26" MTB/tourer. Probably a hard tail-you don't want full suspension bobbing up and down all day long, and suspensions limit what you can hang on your frame. I'd also invest in a good Brooks saddle-maybe a B66, considering that you'll be needing a hardtail.

You might also want to look at a cyclo-cross bike as an alternative. You could even start with thinner tires when you are in the US and change to something that can handle worse roads as the trip progress. You might find a cyclo-cross bike to be more comfortable and a little faster then a mountain bike.