who are the "well dressed" Mexicans the met Carla Jean and Agnes

Ah, ok, I thought you were only referring to the movie. The book differs from the movie in this respect. In the book they didn't need to stake out Carla Jean's house because they tap the Sheriff's home phone. So they hear the conversation between him and Carla Jean when she calls him from outside El Paso to let him know Moss's location.

Which means there's also no moment in the book when a Mexican in a suit intervenes to help Carla Jean's mother with her bags. Her mother does tell the cab driver they're heading to El Paso, but they already know where to go from the wiretap.

I'm glad for your questions, because I think it's an interesting choice by the Coens to change this bit. Cross-cutting from Bell to the Mexicans eavesdropping and peeling out would have created suspense, but at the expense of shock from the seemingliy out-of-the-blue ambush/death, which prompts us to think back to how the Mexicans located Moss. It lets us have the "a-ha" moment instead of giving it to us. The shock the Coens' tactic created calls attention to the theme of "no one sees what's coming."

Anyway, here are the relevant passages:

When he got home she had supper waiting. He dropped the keys to the pickup in the kitchen drawer and went to the sink to wash his hands. His wife laid a piece of paper on the counter and he stood looking at it.
Did she say where she was? This is a West Texas number.
She just said it was Carla Jean and give the number.
He went to the sideboard and called. She and her grandmother were in a motel outside of El Paso. I need for you to tell me somethin, she said.
All right.
Is your word good?
Yes it is.
Even to me?
I'd say especially to you.
He could hear her breathing in the receiver. Traffic in the distance.
Sheriff?
Yes mam.
If I tell you where he called from do you give your word that no harm will come to him.
I can give my word that no harm will come to him from me. I can do that.
After a while she said: Okay.

The man sitting at the little plywood table that folded up from the wall onto a hinged leg finished writing on the pad of paper and took off the headset and laid it on the table in front of him and passed both hands backwards over the sides of his black hair. He turned and looked toward the rear of the trailer where the second man was stretched out on the bed. Listo? he said.
The man sat up and swung his legs to the floor. He sat there for a minute and then he rose and came forward.
You got it?
I got it.
He tore the sheet off the pad and handed it to him and he read it and folded it and put it into his shirtpocket. Then he reached up and opened one of the kitchen cabinets and took out a camouflage-finished submachinegun and a pair of spare clips and pushed open the door and stepped down into the lot and shut the door behind him. He crossed the gravel to where a black Plymouth Barracuda was parked and opened the door and pitched the machinegun in on the far seat and lowered himself in and shut the door and started the engine. He blipped the throttle a couple of times and then pulled out onto the blacktop and turned on the lights and shifted into second gear and went up the road with the car squatting on the big rear tires and fishtailing and the tires whining and unspooling clouds of rubbersmoke behind him.