I don't think you need the solenoid. Lots of people (like me) don't use one. But it would make the co2 last a bit longer. for me it might save $5.00 a year but a solenoid is $20-30.
For needle valve, lots of people (like me) use the Clippard MNV-4k2 available lots of places. Of course there are many alternatives.
Check out my web site for more info on needle & metering valves.