Unless I am much mistaken, ALL metal detectors will detect junk as well as value items.
The user can tune the machine to indicate some items to be junk with a degree of certainty, ie the Fisher Goldbug series, lower frequency machines make a ‘fart’ sound when passed over iron or steel, and this cannot be eradicated. Certain current NZ currency coinage has a high degree of iron in its make up, so depending whether you wish to pocket coins 50c value and below (I do) you might find the indications annoying because until you develop your ear, you will find junk iron and steel as well. NZ ‘gold’ coins are very simple to detect as are pre decimal coins. Beer bottle caps seem to confuse all machines so dig… you might find your lunch money rather than a Speights crown top. Probably the cheapest machine designed for detecting Gold is the afore mentioned Gold bug series, though I am sure dealers of other makes will also advise you, as the Gold bug is aging but it is also simple to use and robust. If you need any more info regarding the Fisher series, just ask.
I have fiddled around with them since I think 1974. Back then they were not available in NZ so dad got two in from Flirida in the USA. A Whites 6000 and a Garrett Deepseeker. They are ancient technology but simple and effective…if you could find one in excellent condition then today they are worth peanuts and a great starting point. Although antiquated they were great detectors. If you want to start off modern then a Goldbug from Robert McBride would be first choice. So simple to use. Next step and an amazing detector might be an Equinox800. I have all the above plus a Minelab Xtreme which I hate and have not used since I got it! For a starter detector and if I was only ever going to buy one then either the Goldbug or the Equinix800.