I agree with what the others say.. also:
Make sure your manifold is of the double oring barrel type (scubapro/DIRzone/Agir Brok etc) - not facing orings - like the MDE type manifold.
Buy the best bands for your twins that you can afford - v4tec, agir brok, highland mills etc - don't skimp on this bit.
Pony bottle - I assume you mean deco/stage cylinder - you don't really need the halcyon mounting kit - you can make up you own rigging kit simple enough (
DIR-diver.com - US Navy Diving Manual pdf.
hth