1st Rebreather
This is a huge and complex issue - I wish it was that easy and a one liner. I wouldn't sit on the fence either.
You need to understand that a first rebreather requires time and effort to develop or redevelop skills of you are already doing Advanced or Technical Diving. You can't take your Trimix Open Circuit Qualification and to a great extent the experience you gain from such and think it is fine and counts hugely to a closed circuit qualification and diving projects - it broadly doesn't.
I do agree that closed circuit qualifications need to be developed alongside open circuit skills - I don't understand the logic myself of those doing far greater closed circuit projects than open circuit skills. After all bailout is to open circuit....
There are many rebreathers on the market some aimed at the recreational scuba market, some at the semi-professional explorer and some the military. All are fit for the purpose for the middle of the market they serve. Many though fall short in one or more areas due to design choices (i.e. counter-lung location either favours exhalation, inhalation or a sort of hybrid to be ok for both). This last point is not a criticism - but broadly sensible commercial design choices. After all the world wide market for rebreathers is not that big, it most certainly isn't as big as you would think, or are led to believe.
I've seen many of these 'breathers pretty much come from a quasi prototyping design methodology. I haven't seen many that have been CAD/CAM designed from 1st engineering principles. To be doing dives to 50msw on a breather is one thing, to be doing dives of 50msw to 100msw is very much another.
The other problem is we want 100msw capability but not a £40k price tag and not something that would task load beyond belief or has design compromises for such a job. The other challange is good old Boyle's Law, using a pair of 3L cylinders for Oxygen and Diluent is ok for the 50msw range. Do your maths this isn't good for the 70msw to 125msw range. So you really need to either have 5L cylinders or even 7L cylinders or better still off-board HP quick disconnects to refill cylinders in water or more conventionally use off-board gas instead. Then there is the choice of Diluent... a single Dil is not a good solution all the way down to 100msw and back... The other challange is you have scrubber life. You don't get much bottom time at 100msw with a scrubber life below 4 hours in cold water, so you have some interesting proceedure to follow for your deco (i.e. Open Circuit or 2nd rebreather with scrubber heating etc or 2nd loop with the same preheating.. or the obvious and go for a breather with a 4 to 7 hour scrubber life.
So my advice would be to get a breather that is rock solid to 50msw and gain lots and lots of hours and experience on this... then carefully look at your rig to then grow it for the deeper dives or buy another breather more suited for the depth. Something like the Sentinel or Inspiration is a good bet for a £5k to £6k investment, Mk15/5, Megladon, CIS-Lunar or the Ouroboros are all good solutions and these all can be someone's 1st rebreather; there are many more to choose from.
Clearly you need to progress your training wisely and slowly consolidating Mod 1, Mod 2 and then Mod 3 training - so by the time your qualified for deep advanced trimix dives on closed circuit you will see the issues and implications - and indeed trade-offs with the designs that better suit your diving needs.
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