I would personally say if I had to spend 600 on my only detector that I would buy a used Minelab SE Pro, just a year or two ago it wouldn't be possible, but lately I have seen quite a few SE Pro's on the market for 600 to 650, and I feel it would be the best bang for your buck. Its a FBS machine with many many coil options, a tried and true detector and enough bells and whistles to keep you going for quite some time. Like I said if I had to spend 600 on my one and only detector the SE Pro is literally the one I would buy. With maybe one accecption, if you are a relic only hunter I would probably go for a used Fisher F75, they are really great relic machines, but you asked for a all arounder and SE Pro imo is the way to go. If you 100 % need to have a brand new machine for some reason then maybe the ATP or 705 would be your route, but I never once had a problem with buying good used detectors.
HH,
Rich