G'day
The problem is that there are "coin and relic detectors" and then there are "gold detectors", neither one is particularly good at doing both jobs as one type of detector known as a vlf (very low frequency) has strengths in the area of discrimination, which is a method of being able to set it to ignore ferrous metal such as tin and iron, and you can still be able to get ferrous metals like silver gold and copper objects like coins and jewelry, these types of detectors are better for use in more benign soil types (low mineral) soils like you would find in parks and dry beach sand, some will struggle with wet beach sand and black sand in places, these detectors can also struggle in high mineral goldfields soils, (hot soil) they will work of course but can be erratic and hard to use, and are not very good depth wise in some areas as they don't handle the hot soils well, some of these types of detectors are able to be used better in hot soils than others as they can come with other search coils and have settings that enable the machine to run more effectively, so if you purchase this type of detector make sure you do some research on the model and ask the dealer to explain how it will have to be used in hot soils, some common makes are Minelab, Whites, Garrett,
The other type of detector that is primarily used for gold prospecting is the pi detector (pulse induction) and again there are many makes and models of these machines as well, there are basic models and top of the range models, but unlike the vlf detectors they are very poor at discriminating out unwanted ferrous metals but are very capable at handling the hot soils and achieving very good depth on targets, so again they are hard to use in areas like parks and fields because these places can be badly littered with ferrous trash, they can be used quite effectively on beaches but they can suffer from another thing called EMI (electro magnetic interference) so any electrical things such as power lines and so on in the vicinity can cause issues, there are pi detectors that are designed for the beaches as well and if you are interested in doing a lot of beach work you can do some research also or ask some dealers what they would recommend, some common makes are Minelab, Whites, Garrett.
There is heaps of information on detectors on the web and dealer sites, but keep in mind that if you only get one detector then you will find that it will not do everything effectively and there will be trade offs, I use 3 main detectors as I hunt coins and relics in some places gold in others, some machines can also be complicated to learn to use effectively so my advice would be to get something basic with minimal controls, some detectors are pretty much switch on and go but others have a multitude of operational choices and can leave you bewildered, price can very much dictate the quality of the detector so get he best you can within your budget.
cheers
stayyerAU