I have never owned any of them, but I read a lot about detectors and I read up on both the Ace 250 and F2 before buying some of my detectors.
Ace 250 is not a real good wet sand beach machine, it has a hard time handling the salt mineralization. In dry sand it would be fine. It can be used in wet salt sand, but turn the sense way down to get it stable.
The F2 is not water proof either, should have no problem in the wet salt sand, but you can't take it in the water with out fear of turning it into an expensive paper weight.
Neither the Prizm 2 or 3 are beach detectors, they work fine in lots, parks and playgrounds but avoid wet salt sand and water.
Detectors are like golf clubs, you wouldn't want to putt with a driver, or Tee off with a putter, it is the same with detectors. It depends on how much beach detecting you want to do. If you want to primarily detect the beaches, your going to need to get a good beach detector. For the $500 budget, you could buy a good used detector off of one of the boards here from a trusted member.
Ones I have used and know...................
A good used Whites beach hunter ID is water proof to about 30 feet I believe and works well on land. I owned one, used it in the surf a lot and liked it, but to be honest I traded it after it leaked on me twice. Usually around $350 to $650 used depending on condition and age.
A Minelab Sovereign is a fantastic beach detector, but again it is not water proof, so one slip and you have an expensive paper weight. But it works great in dry and wet salt sand and is a good land detector. Usually around $400 to $650 used depending on condition and age.
Minelab Excal 1000 / 800 or the Excal II are all good water machines, and all are waterproof to 200 feet.
They can be used on land, but they are heavy..... Usually $700 to $950 used depending on condition and age.
Ones I have never used but have heard good reviews on....
Tesoro makes the Sand Shark and Tiger shark, both are waterproof, I have read the Sand Shark is better for slat water, but I have never owned or used them. I do not know the average used price, sorry...
Garrett and Fisher both make beach machines, but I have no experience on them, other members here do and should respond when they find you thread.
If you serious about beach detecting, buy the best you can afford now.....You will be living close to the beaches and there is a lot of beach detecting available to you.........
Sorry I can't help more...............
Good luck and good hunting...................