Your own "monster garage" reservoir setup
You need 4 things: (1) a tank with two openings (2) a compressor that puts out good pressure (you already have that), (3) misc metal hoses and fittings, and (4) access to welding equipment
It's really simple and certainly doable. The plain tanks you can buy only have one opening. If you go to a welding shop and explain it they'll know exactly what you want and could probably help. You at least need another hole drilled in the tank and a fitting of some type welded to it that you could place a pressure regulator on. I don't know how much it would cost but probably less than a new setup.
I would also weld some type of bracket and stand to the tank. Bracket to bolt on the compressor so it isn't two separate pieces and stand so it doesn't fall over.
Automatic shutoff mechanism is another issue. Basically it's impossible for the tank to fill up beyond the pressure of the compressor anyway so it doesn't matter if it keeps running in that sense.
It's a good project but probably easier (oooh I really hate that word in this context
) just to buy a new setup.
Good luck with it. I think it's a cool project.