Hi, I've owned an MX7. Sold it after 3 months. Nice detector, okay tones, jumpy and annoying target ID past about 4" on solid coin/jewelry targets and fairly heavy/not very ergonomic in my opinion. Really below average target separation and slow recovery speed. 8AAs. No wireless options. Still a solid detector especially in mild soil conditions.
Equinox 600 (the 800 has more options for $200+ more but for your situation I don't know that it is worth the expense???) is in another league entirely from the MX7 and costs about the same. I hunt in bad mineralization most of the time. My 600 will hit solid on any quality targets down to 8" with good tone and fairly good target ID. It will go lots deeper too. Super fast target recovery speed and great target separation in most situations except bed of nails. It feels better than the MX7 (to me anyway), excellent wireless options, (the wired headphones included with the 600 are a total joke BTW), tones are better for target recognition (IMO), numerical target ID is a lot better, full individual segment notching and discrimination and just as nice a display without any gimmicks like words that tell you what the target is and are wrong more than half the time from my experience. The rechargeable batteries need to be refreshed every two to three months according to the manual.
The 600 is a ton of detector with way more options and adjustability than the MX7 plus the simultaneous multi frequency operation and option to operate in single frequencies is really nice. I have both and use the 600 for most of the daily grunt work hunts. I save the 800 for prospecting and special locations.
just my opinions from my experience.
Jeff