Medical tanks have different valves, and usually valve threads, so you either have to go with a scuba setup, with scuba valves, or a medical setup, with a medical valve. Adaptors are available to go from either one to the other, but since a medical tank cannot be used in the water (due the mostly the valve), while a scuba tank can be used in or out or the water, most divers doing what you describe use scuba tanks.
This makes a good use for old steel 72s, 40 to 60 alu, or even cheap bad alloy alu tanks, since unless you have a buddy with a booster you will not be filling them over 2000 psi. How practical using scuba tanks is depends very much on whether you can get 100% O2 fills where you are - most welding/med gas suppliers will not fill privately owned, non-medical tanks.
As to regulators, the easiest and cheapest solution is to just get an old scuba regulator cleaned for O2 and use that. Being a demand system, it will make the most efficient use of the gas, but will only work on a concious, reasonably functional diver. So it is very desirable to have some sort of rig - either a med reg with a non-rebreather mask and an adaptor to use it on a scuba tank, or an adaptor so you can use a non-rebreather mask on your 02 cleaned scuba reg - for treating unconcious subjects.
This is one of those things where you can spend a whole lot of money and still not get an optimal setup, or a little and get a very flexible one, so you got to do your homework.
The big problem with a combo deco/treatment tank is you can't count on it to still have enough gas in it when you need it. So it makes sense to have both a dedicated treatment and deco bottles, but set up so you can use both interchangeably with the regs, so the deco bottle can act as a backup to the med bottle - most med bottles just don't have the capacity necessary anyhow to optimally treat a serious incident (one more big advantage to using a 40 to 80 cf scuba bottle rather than a typical 12 to 22 foot med bottle).
Though if you going to be using the tank in a professional capacity (if you are an instructor, commercial boat owner, or DM) you might want to have a genuine medical setup for treatment, for liability reasons.
