@Dann-Oh - the best rebreather is the one you are using
and have experience of diving.
Assuming from your post that you've not dived rebreathers before and you've no experience with them. That's good; we all started from there
Rebreathers need a LOT of practice and constant diving. They're not like changing from single-tank to twinset to sidemount; rebreathers are very different than open circuit. This implies that you'll need to select one rebreather and dive it regularly for a couple of years or more, especially as you're talking about deep diving (100m/330ft).
You've mentioned three different styles of rebreather: backmount (good for boat/wreck diving and deep), sidemount (specialist, awkward) and chestmount (not good for deep).
My personal advice to you is to choose a common and well-regarded backmount unit. Only when you're experienced with diving a rebreather -- meaning at least a couple of hundred hours/dives -- should you then consider something different.
Common units that are well regarded are probably the ones you've seen on your dive boats already. For example the JJ and Revo. Many others are available.
My diving is on a Revo; I dive in the UK (cold water) predominantly off of dive boats for the past 5 years down to 75m/250ft. This works very well and the Revo has been a very reliable unit which I've no intention of changing. It also works well in overhead environments provided they're not too wriggly.
As long as you choose a common and well regarded unit, it should suit you well for boat diving.
You must budget for the training and practice diving you will need. I've no idea what your current diving is but the Divemaster qualification is of no real benefit. When diving a rebreather, you'll naturally want to dive deep (meaning trimix) and long (meaning substantial decompression). If you've those skills already, then you'll be well prepared for some aspects of rebreather diving. If you've been predominantly diving to NDLs and nitrox; there's a lot more to learn before you can head down to deep and long.
The first course will be your "MOD1" which is how to dive the unit and do some light decompression, possibly with trimix but normally you'd probably get qualified on nitrox to either 30m/100ft or 40m/130ft. A few experienced divers with exceptional skills or prior trimix & deco experience may pass the 45m/150ft MOD1 but they’re the exception.
Once you have MOD1, you must do 100 hours of diving on it in different locations including shallower locations where you constantly run through your skills and get the buoyancy sorted out. Buoyancy on a rebreather takes a lot of time and effort to master and be comfortable, especially shallow 6m/20ft (which you'll be spending a lot of time at whilst decompressing from deep dives).
Only after getting very comfortable in your unit and mastering the various skills learned on your MOD1 will you move on to MOD2. Doing this early is not recommended as it will be a miserable experience (just like doing twinset skills if the basics aren't good).
MOD2 is about decompression diving and staying alive on the unit when you've a considerable decompression obligation. You'll be bailing out to open circuit trimix and practising endless skills for keeping your unit running if, for example, you've run out of gas, there's been electronic failures, etc. MOD2 dives are generally around the 2h to 3h duration with 1h to 2h30 decompression times.
Again, after MOD2 you'll need to do a lot of practice and actual dives to master those skills. At least you'll now be "qualified" to do 70m/230ft dives, so well set to develop your wreck diving experience.
Rebreathers are massively useful tools for diving; incredibly flexible and quite cheap to run in comparison with open circuit. They come at a cost of complexity and greater levels of planning and preparation.
It would help if you've a friend/buddy you could do the journey with. Someone to "play" and practice with is an excellent help, especially when doing the endless skills practice.
Selecting the rebreather is the easy bit!
