Required documentation to visit the US

There's an elderly couple who've lived in Mexico their entire life and have never been to the United States. Their 50th wedding anniversary is approaching, and since they don't have a lot of money, I was thinking about flying them up here to stay with us for a week or two. I have no idea where to start when it comes to documentation required for them to make the trip. They live in a very small town (if you can even call it that) far away from any city of significant size, so driving back and forth to fill out documents is probably out of the question. Obviously, I have many questions, here are some of them:
1. What kind of documentation will they need to complete?
2. How much will it cost?
3. How long of a process is it (i.e., how soon could they visit us?)
4. I want to do as much of the process for them as I possibly can. Can I start by getting the required documents via the internet and mailing to them for completion?
5. Is this whole idea even possible?

I don't know if this message board is the right place for me to be asking these questions, but if someone reads this and knows where I should go to get the right kind of assistance, I would greatly appreciate the information. I've already located the nearest Mexican Consulate's office, but haven't made an appointment yet.

Signed,
Confused, but Hopeful Gringo