Ok, I'm going to jump in here. You've already gotten great advice from very experience water hunters, of which I am not. But, I just went through the research and buying process myself, so maybe I can help. My budget was a little lower then yours, but I already had a scoop, so it's pretty close to the same.
I'm in Florida, and don't have the awful black sand of other parts, but still your average machine won't cut it, so I still had to figure what would help me accomplish my MD goals. I initially wanted to just hit the wet sand out to about knee-deep, so I figured a non-waterproof machine like the Sovereigns, etc or similar would be fine, but also considered the Excals. After much debate, I had my mind set on a Sov GT, but they seem to be harder then hen's teeth to find at a reasonable ($500-600) price. Then I found a Minelab Explorer SE on the forum for $500. After much thought, and getting advice, I bought it.
So now I have a crazy deep machine that can handle the saltwater. Woohoo! Well, after my first run at the beach, I realized knee-deep is where I may be standing, but when you're trying to work the scoop, and keep the machine dry, it can become cumbersome, and the chance of a mishap goes up tenfold. So I quickly realized that a machine with only a coil that is waterproof may not be the 'perfect' beach machine for me. So that kind of put me back to square one.
I'm keeping the SE as my main land, dry beach, and wet sand machine, as I do really like it, but it won't be going out into the water too much. Better safe then sorry.
So now I was looking for a fully waterproof machine, trying again to stay in the $500-600 price range, new or used, more if I felt I could really justify it to myself.
First machine that came to mind was the Excal. There is one for sale on another forum that seems great, but IMO, there are drawbacks to the Excal. From what I've read, there are suggested modifications and improvements to really make the Excal the end-all machine. I don't know about you, but I don't want to get a machine that I have to do things to to get the most out of it. So while the Excal is a great machine, I felt it wouldn't be great for me.
Next up were the PI machines. I have a Tesoro Compadre which I really enjoy, so the Sand Shark was definitely a runner, not to mention the price and warranty are awesome.
The Fisher CZ's that are waterproof are over my budget, as well as the CTX, and the newer White's. Some of the older versions were either not fully waterproof, or didn't have good user reviews (and I read tons of them), or were overpriced for what they were.
The White Surfmaster PI Dual Field came the closest in budget and what people are saying in general about it, and I'm sure it's a great machine, but I ended up getting a Sand Shark.
I went with the Shark for a few reasons. First, a lifetime warranty on a machine that will be constantly in the salt water is invaluable. I read lots of reports (reviews, forum threads, etc) where a submersible machine's housing or housing seal failed after x number of years, and the repair falls on the owner if it's out of warranty. That could become expensive depending upon how much damage was done. As long as I care for the Shark properly, if the housing fails, I get to send it to Tesoro and they get to fix it.
Price was also a major factor. Money is not the issue, but value is. For $600 I get a brand new machine, and not have to wonder how it was cared for by anyone but me. Seals dry out, crack, coils get banged, machines get dropped. With this one, I know how it's been treated it's whole life.
Reputation. The Shark has an excellent reputation, as well as Tesoro's customer service and repair. I know most (all) of the manufacturer's have great customer service, especially if you bought a unit new, but it was the nightmares I've read about with some repair centers taking months to make a repair that had me shy away from some.
Parts and future serviceability. There are a number of (recent) threads discussing parts availability and manufacturer support of discontinued machines. I may never need some weird part, but what if I do, and they tell me that part is no longer available? A MD makes an awfully big paperweight, and doesn't weigh enough to be used as an anchor, so...
I have absolutely no qualms about buying a used machine. I hardly buy anything new if I can get a great value on the same thing used. But the thing I noticed about metal detectors, is that a used machine holds it's value very well. To me, the minimal savings isn't worth the potential headaches down the road.
Now. Based on what you've posted about the sand conditions you'll be hunting, I would look for a PI machine, with my suggestion being the Sand Shark, UNLESS, as already stated, it's a really trashy beach. In that case, I would look at the White's Dual, but I would look for real feedback on how well it really does on a trashy beach. Or I would try for a used Excalibur, or a CZ-20/21.
If the beaches you're targeting are generally people-gathering beaches, I would guess they aren't too trashy, especially not so bad once you're in the water.
No matter what though, I will stress again that a machine being able to be run in the wet/water is only one side of what you need. The other side is really the waterproof ability, seriously. Besides, even if you plan now to only go knee-deep, the water will be calling you every time you're out there, and with a fully submersible unit, you can answer that call!
Let us know what you end up getting!
HH