I would strongly suggest you carefully look at the Exclerator coils. Either SEF 10x12 or 12x15, or the 10x14 DD. Any of these 3 will still retain the ability to find very small objects. Not sure about the larger 18DD available, but the first 3 listed would be very accurate, sensative, deep and cover a lot of sand per sweep. The Big Foot is a great coil for coverage and target recovery upto 6" and is not intended for deep seeking.
All 3 coils are available right in your back yard at Kellyco for under $200 (the Big Foot will cost you $250 if a new one is even available or upwards for $350 if they are still out of production)