Just go with HP, there are only a few markers on the market that require a LP reg on the tank (Angels). If you shoot fast enough with a LP tank, you will start to drop off and choke.
Stick with a HP tank.
And Hydro is the type of testing used to make sure the tank is still safe. Still in Hydro, still safe, not in hydro, potentially dangerous.
Oh, I think RB has some HPA tanks for sale here in the Air Systems section.