No problem, you need to buy a special sticker if you plan on driving in to Mexico but if you stay near the border it's not required. Just drive 20-50 miles south and the Police will stop every one and check for documentation, sticker, dead people, drugs and any reason to improve the odds for a bribe. I'm not sure about car insurance.
Mexico uses unleaded gas just like USA but usually some one fills the tank for you.
Choose hotels that have gated parking with security.
Oh, almost forgot that now you should have a US passport if you expect to come back with no hassles from the INS.