For part of your questions - the GTI & GTP series as they have automatic ground balance and ground tracking ( measure the mineralization and adjust to it), Scan Track whereas the detector adjusts to your particular method of scanning. The older GTI's can be equipped with the new "chip" and the new ones since a few months back come with it ( requested by the folks in Australia ) that allows them to master the toughest mineralization. This should also render them better equipped for salt sand but I haven't heard any reports yet. I'm going to have my 2500 equipped with the chip,check it out, and report on it.
For deep caches ( measured in feet ) the 2500 and CX series can be equipped with the Treaasure Hound.
For older coins the 2500 in all metal should get you down there where everyone thinks they are, but all old coins are not buried halfway to China. I've found coins from the 1700's at two inches and clad at 8-10 inches in the same area. For prospecting the 2500 will work well ( especially with the new chip )as will the Scorpion Gold Stinger which is specifically designed for prospecting.
The PI's are a breed all their own and can be used for relics, beach, and underwater hunting - the Infinium down to 200 feet. They aren't the best for dry land coin hunting unless you plan on digging everything. But they will go extremely deep.
For just sheer, flat out, throw it in your car fun coinshooting, the Ace 250 is really hard to beat, and it will hold it's own with machines costing three to four times as much. Garrett has a detector for just about any type of hunting you have in mind and some to cover several different phases of hunting. Hope I have answered some of your concerns.
Bill