If you are talking about importing a vehicle from the US into Mexico there are restrictions. The one that killed us is that the vehicle can not be more than 10 years old. We sold our classic VW in May and it hurt me.
The ULSD question has been discussed very thoroughly under the heading Mexican Diesel Fuel-Sulphur Level Updates which is on this page. It should answer all of your questions regarding fuel.
When we were shopping for a vehicle here we tried Mercado Libre, but didn't want to have to travel across the country to get what we wanted. We finally found that there is a Facebook page for vehicles that you could sort into just your area. Once we found that, we found exactly what we wanted and here. It's called Marketplace. It makes using Facebook worth while!