Hi JBM;
I assume you are primarily a cold water diver/dry suit diver. Selection of the HP120 would tend to indicate as much. These tanks are heavy as lead and rather long but represent a good selection for a bully, fit chap who is diving in rugged northern conditions. They are not suitable for tropical diving. Besides dry suits, the 120 tank can be used with thick wet suits but in either case be sure your BC is reliable with a rugged fabric. Swimming up from one or two hundred feet with a deflated BC can be iffy. The suit selection may impact your BC choice as to back inflated or vest and for other reasons also. Vests(jackets) are good for wreck divers because of the pockets, room for a doubloon or two. Back inflators require a swarm of hooks and rings or awkward accessory pockets to secure stuff. Also, they may become somewhat irrelevant on a surface swim. If you are doing something that requires an unobstructed chest area, like cocking a big speargun or operating controls on a drysuit, the back wing might be for you.
I doubt you need to spend the money on an ATX200 but you may find you like the adjustable second available on some Apeks or other brands. Having an adjustable first and second stg gives the advanced diver more options. The sealed first stage is the way to go irrespective of brand or model. It should be pointed out that the usually cheaper Aqualung regulators are available in cold water versions. The difference in the brands is more "mental" than anything else. Selection of a specific backup is not crucial; a simple, cheap unit is OK but in making the selection ensure that it is specifically marketed as a backup second with detuned action. These are less likely to freeflow.
The Genesis "react" is a really nice computer, good graphics and user replaceable battery. Suggest you consider it as a console mount.
Consoles feature Rubber boots and these are necessary to protect the delicate action of a bourdon tube pressure gauge(dial gauge) or magnetic compass, if used. My preference is for a wrist mounted compass because this item, when needed, may have to be referred to frequently, in which case no boot is required. Just don't drop it on a hard surface.
Good luck.