You can change a 3 phase motor to a single phase if it is around or less than 7 hp (There are also phase converters available, but they draw more amperage). For what you are doing I would recommend at the least a 10 cfm compressor. That poor thing will run for days otherwise. Auto drains and simplicity would be top of the list for those running the compressor. One person doing it could easily get tired of filling tanks/banks quickly, especially if pumping nitrox which requires a little more care in making sure the mixture is on, and there is O2 in the tank. Filtration should be top notch too, and may require adding a tower filter into the system. A CO meter should be considered mandatory! Analox and Nuvair both make ones designed to shut a compressor down and trigger and alarm. 13 of us got CO poisoning from a failed compressor, it is a real threat.

How many people are going to be getting fills and how often? Do you have an approximate tank count per month or week? Four 4500 psi banks filling high pressure tanks is not going to go very far. Of course that also depends on how full the tanks are when beginning the fills. Having around 6 3,600 psi banks would help extend the higher psi banks filling capacity, if used as a cascade. Of course a booster could resolve some of those issues and might be an excellent addition (Wish I had a booster).