... The idea is to go to 3 meters depth ....
That's nearly 10 ft. ! Even with your explanation following, I'll have to be a kill-joy and say, that anytime someone comes on with a "spanish treasure 10 ft. deep", it's almost always superstition and treasure-legend ghost story stuff. The giveaway is the incredible depths (and usually millions of dollars in gold) that are "bullet proof and iron-clad true".
For example, you say:
... Some of the homes here that have been remodeled have had caches of gold bars, jewelry and coins. Some in the old adobe walls and some under the foundations.....
So too did I hear of such fabulous caches found in walls, floors, etc... of the places my host was going to take me to in Mexican towns of the Sierra Madres, that date back to the 1600s. Thus *certainly* if const. worker, or gardener, etc... is finding these things by accident, then certainly a detector would make child's play of finding more. Right ?
But when we got down there, I asked my host to introduce me to some of these various people he knew of, that had found these caches. And one by one, the stories (which seemed SO iron-clad in the USA) fell apart. Invariably, you talk to the person, and ... it turns out ... they didn't actually see the coins. But they got it on good authority from the person who saw them. So you track down THAT person, and ... you guessed it: they didn't actually see them either. But not to worry, because they heard about it from their uncle Miguel, who heard it first hand from the construction worker. So you finally track down THAT person, and ... you guessed it. They didn't actually see the coins either. But not to worry, because they got it on good authority from their buddy at the campfire while they were all at a party 10 yrs. earlier. And on and on and one it goes. You can never actually see the coins. Yet in their mind's eyes, this is "first-hand info" still. They simply can not see the telephone game going on.
So too do I bet you've never actually seen coins found in such a fashion. You've heard about it, and you have no reason to doubt those that told you. But if you start digging deeper, you'll see the stories fall apart in the same way as I describe.
And I don't get the "3 meters" stuff: If the stuff is in walls, then walls (even of thick adobe) are rarely more than a foot thick (in-so-far-as the wall -hunting side of your story goes).
If you're still convinced there's treasures waiting for you in all these places, get a 2-box unit.
We found individual coins back to the 1850s in Mexico, but no caches. Had a lot of fun, but I came away with a big dis-trust of all the treasure lore that fills Mexican culture, as nothing but camp-fire stories gone awry. And humorously, even when you show them that there no metal (treasure) in the floor of a cave, for instance (because you pull out your headphone jack and let them listen for themselves that it's utterly sterile in there), that still doesn't convince them. That doesn't mean there's not a certain treasure there. It merely means you need a detector that goes ...... you guessed it .... 3 meters deep. Aaarrrgghhhh