Talking from experience - best and cheapest:

- Get a double gauge CO2 regulator from e-bay.
Look for the Beverage Factory regulator. $45 including shipping.
Ordered 2 times from them - great service both times

- Get A GOOD needle valve
Forget the $5 Home Depot. It does work but it feels and adjusts like
crap.
Go for the needle valve that Marine Monsters used to sell, may stil
have it. It's a valve made by Clippard. Floridadriftwood has it -
will be $30 or so with shipping. Too expensive, but that's the best
valve out there for now.

- Get a check valve.
A cheap one = $.99 from any pet store will do for long enough. Later
get a metallic one.

- Get a full CO2 bottle from a local gas or beer brewing shop. Got to
be $50 - 60 for a 5 lb. bottle. When transporting - have the bottle
upright in the trunk of your car, not in the car with you.

- Forget CO2 reactors or diffusers. Stick the end of the CO2 tubing
into the canister filter intake.

Attach the needle valve to the regulator using a 1-1/2 inch of clear
hose (Home Depot). Just slip it on the barb that the regulator comes
with and slip the needle valve on the other end. Leave the whole
thing in the room, NOT under the tank for 2-3 days to make sure the
bottle does not release the gas tru the safety valve for some reason.
Done.

Total cost - $120-130. You will have the best out there for the least
money.
Tried and true.