I'm in a similar situation, wanting to replace a Conshelf SE2 with something newer, and I'm also rigging it DIR (single tank). I don't have an answer for you, but here's some things to consider, or at least what I've been mulling over. I'd appreciate your feedback too.
1) I decided against a 1st stage swivel (turret) to eliminate the dynamic o-ring there. I think the Mk25 uses a 1st stage swivel.
2) I decided against a 1/2" regulator hose (1st -> 2nd) in favor of the more universal 3/8" size. That would mean plugging (or adapting) the dedicated port on some regulators (e.g. ATX200).
3) Are you planning to setup for a single or twins? I don't know for sure but the hose routing concerns are probably different for the two configurations. There's some nice pictures here
http://www.wkpp.org/articles/Gear/equipmentimages.htm of a TX100 twin setup, looks very clean.
4) I'm still debating the TX vs ATX 2nd stage question. I've read that the larger TX 2nd is somewhat better than the ATX for bubble interference, but I don't know that for a fact. Also for 2nd stages, is the adjustment on the ATX50 worth having, or would the ATX40 be a better choice? I'd prefer not to have a 2nd stage adjustment if possible.
5) If I was buying today, I think I would get a TX50 with DS4 1st stage and an ATX40 backup, probably from
www.simplyscuba.com. Unfortunately this combo costs more than getting an ATX50 with the DST 1st, since Apeks doesn't offer a regulator set with the DS4 1st, making this a custom order. I've heard that the DS4 breathes as well as the TX100 and is considerably smaller, lighter, and less expensive.
6) My choice in the Scubapro would probably be the Mk16, again to eliminate the 1st stage swivel, but I maybe over reacting to what might be a useful feature.
Anyway, those are my thoughts today, but will likely change.
Don
