Before I go anywhere I'd rather know the plusses and minuses, and it surely seems you have experience in traveling to exotic places, so keep your eyes open and enjoy yourselves. I look forward to reading about your trip.
Living in San Diego for over 52 years I have been deep in Mexico more times than I count. Two day stays to extended trips, most trouble free! most! Mexico is a beautiful country, inland and on the coast. However until things are safer I will not take my family there, that's just me. I believe our own government is temporarly discouraging travel to Mexico. It's only a matter of time before Mexico will again be safe. Make sure no one in your group takes a gun, it's illegel and a serious crime.Wish all the luck to your group.
Have some minimum Spanish available; it IS their country. We've been there; it's great! Hope the beer in Yucatan is better than when we were there: tasted like a Doctor's office smelled.
Mechanics in Mexico, etc., can fix anything.
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1983 310 Turbo Diesel Mo/Ho "Bess" 212,292 miles
Full timing since 1994
Spare parts? How about plenty of toilet paper, bee sting sticks, water purification device and tablets, extra electrical wire, butt crimp joints, plenty of black tape, a good GPS with Mexico and Belize maps, a satellite cell phone. There are actual personal distress signal devices that can be activated if caught in an emergency situation. They send lat/long coordinates to whichever service is monitoring the device. These devices are required on some mountain climbing expeditions.
It may seem like a bit of overkill, but that's why I drive a 3/4 ton Duramax as a TV.
A water filter that will filter out all pathogens. You can get them at stores like REI that are used for camping. You can also use iodine tablets that will do the same thing, but I understand the taste is awful. Some basic medications for water borne illness such as some antibiotics. There are many medical kits available for travel.
Sorry if this is highjacking, but if you consume pulque while in Mexico, you better not have your keys in your pocket. That stuff will have you seeing triple in no time.