There are concrete vaults, plastic cisterns, a lot of choices. Septic tanks are a different matter. You want only something from a licensed supplier, and it should be a 2-compartment tank. The smallest size is 1000 gallons, overkill for a system with a constructed wetlands for graywater treatment. It sounds complex and expensive.
Most people do the work themselves.
I can do the design and help for an hour or for a day, and then let them do the rest. It depends on how much water you get. By the way, the systems do take up some room. Design the house around the system. You want to keep water fixtures close to the constructed wetlands to keep an adequate drain slope.