If you are looking for gold in Australia in highly mineralised ground the best machine will be one set up for this search.
Easiest to work are as follows.
SDC 2300- turn on and go. Resale value is high.
GPX 4500 - takes some practice, loads of settings but usually factory will work loads of coils and big history of gold searching. Resale value is high.
GPX 5000 - takes some practice, loads of settings but usually factory will work, loads of coils and big history of gold searching. Resale value is high.
GPZ 7000 - Expensive, heavy but certainly does the job.
I would not also rule out the QED, take some practice but getting a reputation for gold searching.
The Garrett gear has loads of people using but I have not seen too many using widely on gold fields. Resale value is also low.
It really comes down to budget and what you are searching for and location.
Cheers HH