I used to be the only one who built his own Co2 system but recommended retail. Why pay twice as much when you don't have too? Beside the quality and looks, they both do the same thing right?
A friend of mine started this hobby and got the Milwaukee, and surprise surprise, he too was faced with the same problems. His bps is day to day, and I can't imagine trying to grow plants when your Co2 supply is up and down.
But here the kicker, there are people with working retail kits. There are people with Milwaukee regulators who have great tanks.
Buy a retail kit from a source that has an easy return policy. Try it out, and see if you like the performance. The only hassle free method is buying, or building a Co2 system with parts made for scientific and medical research.
Here is what the "average" price for parts you need:
Regulator: 60 - 100
Needle Valve: 20 - 50
Fittings: 10 - 50
Solenoid: 20 - 60
Also, expect some sellers to label parts as "new old stock" which is basically "old, but I clean it with a rag".
To give you an idea on what condition regulators are sold as:
This regulator was label as used (honest seller) and if what you're buying looks worse than this, then it's super used.