Welcome to the forum
This is just my humble opinion... and consider that I own an ACE 250 and an AT Pro
If I were choosing between the ACE 250 and ACE 350 today. I would go for the ACE 350. I've had the ACE 250 for years and there was no ACE 350 back then.
It seems to me that for the extra $75 cost of an 350 you get a better value (a little better and more modern machine). The ACE 350 and it's DD coil can also be a better choice for a variety of soil conditions. (I do love, and still use the 250 all the time, and I think you'd be very happy with either). The ACE series are really nice general purpose detectors and hard to beat in their price range. They are also just fun and simple to use.
Although I've been in this hobby for a long time, I'm still in it for a fun, and exercise. It sounds like our detection goals and hunt locations are very similar.
The ACE series will work everywhere you want to go with some limitations. I use the ACE 250 mostly on parks, athletic fields, tot lots, beaches and in the first year of owning It, found about $375 in coins (so it paid for itself). The ACE machines are not waterproof but the ACE coils are waterproof and I've even "carefully" waded fresh water beaches up 2 my knees with the 250 and it worked well.
Salt water beaches are where the ACE machines lose it. They work fine in dry sand at salt water beaches but when you try use them in wet sandy areas they false all over the place because they are not (and can not be) ground balanced for salt water. Don't get me wrong, you can still find stuff but it's tedious. The ACE 350 with the DD coil should offer an improvement in this environment but not having used one, I can't really say. There's probably a load of you tube videos with people using the 350 on beaches. The DD coil should also help you if you are hunting where the is black sand or other mineralized soil.
The ACE 350 comes with the 8.5" x 11" DD coil which is very nice and gives good depth. But that's just the beginning LOL. I'd be willing to bet that if you like the hobby, based on where you say you want to hunt, you will be shopping for a smaller coil (or coils) within a year. That's not a bad thing, but as you learn more you will find that smaller coils make picking out good targets in trashy areas considerably easier. If you start out as I did and dig just about everything you'll be rewarded and separation won't matter very much.
Since you had asked about alternate coils.... My ACE 250 cam with an 6.5" x 9" concentric coil and I also purchased the larger 9" x 12" concentric. After using them both, my favorite all around coil is the 6.5" x 9" because it makes the detector a lot lighter and gives me adequate coverage and depth. My 2nd favorite is the 4.5" sniper coil for working tot lots and real trashy areas.
The AT Pro came with both an 8.5" x 11" and 5" x 8" DD coils and my favorite by far is the smaller 5" x 8". I very rarely use the large coils on either detector.
Coil choices are really about personal preferences. As a Rule of thumb, large coils provide larger coverage and go deeper and smaller coils trade off depth and coverage for easier target isolation and getting into tighter places. And if you hunt for long periods of time, weight & fatigue can become a factor. Folks tend to like or dislike coils based on their own hunting goals. As for me, I seem to prefer the smallest coil that can get the job done. I'm not a relic hunter and find that most of my good finds have been down 4-6 inches give or take a little.
Happy Shopping & Hunting
