You're questions are very subjective, and will depend on the individual.
I have the Teknetics G2+, same as the F19, just a different handle setup. IMHO it is VERY good at what a 19KHz detector should be good at. It is extremely sensitive to low conductors and tiny targets since it's origins are in the gold nugget hunting realm. It does very well in environments with high iron, which has made it a favorite among relic hunters. It does quite well coinshooting, but will loose a little depth compared to some other machines on high conductors like quarters and deep silver, as that's not what it's primarily designed for. Has a very accurate TID.
Depth as with any detector depends on soil conditions. If pure depth is what I'm looking for I'd use my F75.
I find myself using my G2+ more than my F75 or CTX lately, except when I'm looking for deeper high conductors. I hunt freshwater beaches, and sports fields with it using a NEL 12"x13" Tornado coil and it works very well. The standard 11" biaxial coil separates well.
It has true VCO modulated audio, not a specific number of tones. You can adjust the V-Break audio at a point where below the set point will report a low tone, anything above will report in VCO audio, with a higher pitch and volume for large or shallow targets, a more subdued volume/tone for deeper or smaller targets. I really like it!
As to current used prices. I'm seeing them around $300. A new G2+ is currently selling for $449 direct from Tek.