Good luck with this.
I lost a treasured knife this spring, and spent the best part of 2 days metal detecting for it, with no luck.
However! I took the precaution of going armed (this was Fife, after all), and shot 2 roe bucks in the process.
If you do get a machine, make sure you get someone who knows it well to show you how to use it - it is NOT trivial!