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buy an air tank or a new compressor?

I don't rely on any dive or paint ball shop for filling. Their equipment breaks down or they go out of business you're stuck. Get a good compressor and a used scba tank and you're good. If you get a yong Heng stock up on some replacement parts such as cylinders and pistons and burst disks. If the tank is still inside the 15 year life get it hydro'd but if it's outside the 15 years pull the valve out and inspect the interior. Remember, Europe let's their tanks go 30 years so as long as the liner isn't corroded and the outside isn't beat up you should be good. While you have the valve out remove the straw that extends into the bottle. Without that straw all you have to do is turn the bottle upside down and crack the valve knob slightly and any water buildup will be gone.
 
I hand pumped my Prod at first after getting my YH. I was worried it would fill it too fast and I would over fill it. It fills it fast but not that fast. I think it took at least a minute, might have been 2. It fills my 45 minute SCBA tank (I think that's 66 cubic feet) from ~3000 to ~4000 in about 20 minutes.
I've done a similar thing with my SCBA tank for filling guns. I have a M-rod and it does fill pretty quick off the tank but I can control it. OTOH I have a Diana bandit with only a 50cc reservoir. I always hand pump that BC I'm concerned it will fill too fast off the tank and overshoot max pressure.