Gleaner after being involved in the R&D, technical, and detector repairing for many years - here is the correct answer to your questions;
Using "test gardens" and/or plastic or clay pots to test discrimination abilities, recovery speed, depth, or metalic type differences, etc is a complete waste of your time. It's a pish-poor way to test a metal detector.
Once soil has been disturbed or moved, the "matrix" (AKA soil stratum linearity and magnetic flux, direction, etc) is completely in void. It changes, meaning that the molecules, electrons, and stratum are completely at random, and unable to conduct both electricity and magnetism as well as in undisturbed soil. In other words it is no longer going to give you a real-life true rendition of what you would normally find in the field. In most cases soil moved from one place to another will more often than not give you complete opposite readings than you would get in undisturbed soil.
Additionally, the myth of coins giving off a "halo effect" is a complete misnomer and mischaracterisation too, meaning that the "halo effect only applies to FE, meaning iron, steel, and other concentrated iron ores such as pure hematite, etc. Copper to some degree does leach a bit of ionic and/or covalent bonding (ions and/or electrons) with other elements in some certain soils, but that is so negligible that for one thing it would not register any sooner than more than a hundred years, save for when found in soils which have an emense amount of salt in them. And no, even sea water seldom has this much salt, but the Bonneville salt flat does - and in short order it could actually vaporize a copper coin, but the same can't be said for brass or bronze coins.
When a coin has eroded to the extent of it starting to fall apart and break up and scatter about, it's not experiencing the so-called erroneous "halo effect" that people speak of, but even THAT behavior takes decades and decades (100 years+) to accomplish. In some EXTREME CASES some copper or (remotely possibly) brass coins begin to break up in just 30 or so + years but that is so rare that it is nearly a miracle when it happens. If someone had lost a coin in some extreme saline or exteme high acid soil this could happen, but that is extremely rare.
When it comes to the real life world of metal detectors the real truth is that I have metal detectors sitting right here almost 30 years old that compete with those that are brand new on the market, especially as far as discrimination. The in-ground depth varies - - from the same depth in highly mineralized soils to only 2" max in salt beaches and scarcely 1-2" better in mildly-mineralized soils.
Good luck if you buy the Etrac. My gut feelings are that at first you may shortly wonder why you paid the big bucks for it, but in the end you may be like most Minelab users and have to love the thing anyway, since the resale is roughly little more than half what you paid for the thing. Or you may wind up actually almost worshipping it because what else could you do? This phenomenen is akin to the derangement of electing a "popular" or well-liked person as President when he really doesn't have a clue as to what he is doing. Get my drift?
"Try before you buy" is the best way to chose a metal detector, even if you have to drive 800 miles to do it.
Just the thoughts of this old battle relic of an detector engineer, technician, and repairman.
I hope I helped
LL