You have received responses from several SB members whose opinions are to be highly regarded, and whose experience is considerable. Some responses have (not altogether surprisingly) questioned the sincerity of your OP and the reasoning behind your opening statement, and have shared with you their concern that perhaps more diving experience would serve you well before you take the plunge, etc. While I echo some of the sentiments, I choose a slightly different approach to addressing your questions. Let's asume that your OP was sincere, and that it was perhaps more the syntax of the post, rather than the substance, that led some to (? understandably) believe it was a bit flippant, and therefore not to be taken seriously.
One approach to addressing your questions is to tell you what we/I might use to dive to 200'. The last time (not the first time) I did go to 200', in June, my exposure suit was a trilam drysuit (with a separate neoprene hood), I used manifolded double HP130s on my back that were filled with trimix (18/40) and were attached to a SS backplate, with a 60lb single bladder wing, I used Apeks Tec 3 regulators on my backgas tanks, I wore stiff paddle fins (Jetfins) appropriate to pushing me and the heavy rig through the water, I carried two decompression gas bottles (80cf and 40cf), filled with 50% and 100% oxygen, I carried a canister light, and two back-up LED lights, I had three computers (X1. Helo2, Nitek He) strapped to my arms, I carried a 100 lb lift bag, a 140' reel, two cutting devices, two written copies of my dive plan / run times, with contingencies. That was the rig (and I am probably omitting some items), but that was just the 'small' part of what I needed to dive to 200'. I dove with a buddy with whom I have been diving for 5 years, someone who I know well, someone I trust, and someone with whom I communicate effectively underwater. We planned the dive together in advance, we agreed on the plan, the times, the gases, the procedures, and discussed a variety of 'what if' scenarios before entering the water.
And, that part was bigger than the rig, but not the biggest part. Before diving to 200', before diving to 180' or 160' or 100', I trained - from OW ultimately technical training and trimix training - and I dove - lots of dives, lots of decompression dives to various depths shallower than 200', and I practiced - buoyancy control, gas sharing, gas switching, lift bag deployment, no mask swimming, etc., etc. So, I had the training, the body of experience, and the dive buddy that allowed the rig to work for me and allowed me to feel comfortable going to 200'.
Now, having given that more ponderous background, I will also say that a year ago in Bonaire, two other technically trained, experienced, familiar divers and I dove to 170ft on single AL80s filled with air, in westsuits, with no redundant gas supply - other than each other - no a priori dive plan with run times, no decompression gases, and each carrying a single computer, a single cutting device, no lift bag, no reel, etc. We did that because we knew each other well, we were diving in warm, clear water, we knew we could do it, and honestly we did it on a whim - we descended on the outside of the second reef at a particular spot and just decided to follow the descending bottom contour until we more or less mutally agreed to turn around and ascend back to the reef and hang out watching fish for a while.
So, what do you need to go to 200'? You can do it with the same equipment you use to dive to 50'. And, you might / possibly / probably will be successful. But, you can also plan for a drysuit, double tanks of a size appropriate to your a priori calculation of how much gas you need based on your measured consumption rates, and a variety of additional paraphrenalia that will maximize your chances of a safe return, even if the dive goes sour. The choice of gear brands and models is very individual and there are lots of SB threads on the 'best' - drysuit, regulators, backplate, wing, fins, tank, computer, decompression software, fins, undergarments, etc., etc. - you get the picture. And, the gear will maximize (not guarantee) the likelihood of that safe return because you have also pursued the training that will allow you to calmly, comfortably, efficiently deploy the necessary back-up resources at the time they are needed. This is not meant to be a sermon, just a first person testimonial that you can accept or disregard as you choose.
He's been to 850' on rebreathers a few times and routinely does 400. I'm nowhere near that but I'm ready to shoot for 200 on open circuit.
I am curious - 1) where does he routinely dive to 400' (not a cynical question, just genuine curiosity, as I find it hard to identify readily accessible sites that offer that depth), and 2) what tells you that you are ready (again, not in any way saying you are or aren't rather just asking about your thinking).
I'm looking for a good deal on a drysuit. Wouldn't hurt to have a cold water reg. Any good quality off the shelf reg or something else? Aside from the obvious what are the details? Double tanks, what size? Two computers? Three computers? Seventeen computers? Bailout bottle? What else?
I have described what I use (FWIW, I think a good deal on a drysuit is buying one, for whatever price, that fits me well and doesn't leak too much), and what I did to prepare, and leave it to others to do the same.
