Hello Oscar,
The White's PI Pro or double coil are both nice hobby detectors. How deep does it go? Of course that depends on too many variables to answer. The Eric Foster detectors of a few years ago were very deep penetrating machines (I have one of his Deepstar units). The White's TDI is also a deep penetrator. Questions I have for you are: Where are you going to detect (land? in water? saltwater?) What are you hoping to find? (a deeply buried treasure chest? a gold coin? Elvis' Cadillac?) Detecting depth is part transmit power, part directionality of the coil, part coil diameter & number of turns, part detection technology (pulse induction, VDI, etc.), part receive coil and signal amplification circuitry, and one heck of a lot operator skill, familiarity with the machine, and luck. If you are after large targets, a two-box detector system or even ground penetrating radar may be the answer. If you are after small targets at significant depth, I am aware of no hobby detector that will help you. Maybe get a group of college kids from an archaeology class to dig and screen? Just kidding.