Going to buy a metal detector

If anything one of the two box types, but I'm not sure the depth range even those can handle. I'm sure someone here can give you more info though. Good luck.

Todd

At that depth you'll need a two box detector as stated by Todd. The Gemini III is a good one. The targets will have to be of some size to find. Mason jar or larger at that depth. Single coins will not be found by a two box detector at any depth. Le t me know if you need more help.

3 metres (metre = 3' 3" approx.) is 9' 9" not eighteen feet.
Unless he was the local drug smuggler and had crates of coins I can't see any two box being of much use. The Deepers P.I.'s that are made in Mexico can be used for small caches near surface or larger, deeper ones. He may well be able to hire a machine and unlike most P.I.'s it does have some discrimination.
Check out
http://www.metaldetection.net/english/Deep_metal_detection.htm
First thing to do is find out what the coins were buried in. If it was metal boxes then no discrimination should be used.
Its suprising what does get buried when banks are not trusted/times of war etc