I have one of the second gen shoeboxes. Seal replacement is easy. I wouldn't recommend filling a big tank with it. (I have re-filled my 3000 psi scuba tank, but it takes a long time and is hard on the little guy). Filling the small tanks on the airguns is easy for the shoebox though. Stop it every 10 minutes or so and add grease to the cylinder shafts, and away you go until filled.
You can purchase the Omega compressors from AirgunSource, but that is a lot of money just for shooting. IMO. Think my shoebox was around $650 delivered when I ordered it. Not sure what they go for now.
ht tp://www.airgunsource.com/omega-super-charger-4500psi-compressor/dp/1882