Andy, having lived and bought and sold motorcycles for almost 20 years here in Mexico, let me give you the following thoughts.
Bikes, meaning motorcycles different from scooters or pizza delivery bikes are much more expensive than in many other countries.
The selection here has always been limited, for example, the Africa Twin which is a near perfect bike for Mexico was never sold here, but it was in neighboring Guatemala so go figure!
Selection will be limited and accessories hard to find and likely quite expensive.
Luck plays a major part in this. You are in a good area to find something as the Federal District and the State of Mexico are home to well over 20 million people. The bad news is that about .000001 percent of that population will know what you are looking for. But it is a start.
Be very careful with you cash, and also who you trust!!!!! You will have zero chance of ever seeing your money again if you screw up.
Start looking at this page for some idea of what is available in the area where you will be. The area around Guadalajara sometimes will have better options for what you are looking for but that's a fair distance from Mexico City. http://motos.mercadolibre.com.mx/doble-proposito/
I don't recall the really big Honda XR's being sold for any stretch of time in Mexico. Start thinking about a KLR, or maybe a Suzuki DR400 as you might have more luck.
Remember, parts can be hard to get for bikes that were not sold or supported in Mexico.
Remember, many of these bikes that are uncommon in Mexico have been "imported" either legally or illegally into the country. This can be a potential mine field and major headache and expense. If the bike is illegal in Mexico you will have a hard time plating it. If the bike is legal in Mexico but the yearly tenencias or road tax have not been paid you will be on the hook to pay every single one that is owing before you can plate it. You need to know what you are looking for when it comes to this. Many imported bikes in Mexico have a gray background or dubious past and it is easier for people here who know the system to get things done rather than someone who doesn't know the system and is an outsider.
Do you speak Spanish? Can you read Spanish? Are you aware of what you are getting into? Be honest.
Ok, if you are going to see it through, contact the rally organizers for suggestions for local bike shops that might be able to help you. It is better to have a few people on your side and looking out for you here rather than just being another gringo who is ripe for pickings.
I am being blunt, but seriously, buying and selling bikes here is a fool's paradise and you will need some help.
So start looking at what is on the internet market, not everyone will be out to fleece you but be aware you have basically zero consumer rights in Mexico.
Garry Dymond might be able to give you an opinion or two, also, and he likely will be a good contact for you.
Best of luck, I hope to see you on the rally if the route brings you to Veracruz.