Fisher Gold Bug or GB 2, Tesoro Lobo, Garrett AT Gold, White's GMT or the MXT/MXT pro for a 3 mode detector. All are going to be in the 7-800 range new.You can get a deal at most of the GPAA gold shows for around a hundred off list most times. Also look on craigs list for used units or ask at a vendor if they have a trade in they would deal in. Talk to dealers about what you want the machine to do, and where it will be used most. That can narrow down units for you. If they have hands on demo units, that can help as well. Use a figure of 1 inch of depth per $100 as a guideline....if you buy a $700 unit, look for 6-7 inches depth, but that can vary with target size and ground mineralization.
When we bought ours, we were between the Lobo and the MXT (pro was not out yet) Since we are in Oregon, not that much nugget shooting, so we went with the MXT as a general all around unit, knowing we were giving up a little on the gold unit side of things, plus the factory is 12 miles from the house....
No help on the other part of the question, have never done anything but rock hounding 50 years ago down there.