It really depnds on how complicated or how simple you want to go.
At the minimum, you'll want a regulator (2 gauge is best), a tank (5 pounds is the most common), a needle valve (starting at about $18.00 for a decent one), a bubble counter (you'll want some gauge of your flow rate) and some way of putting the CO2 into your tank.
I'd recommend an all in one regulator, by the time you buy a regulator and a needle valve, your at the point where you can buy an all in one unit (regulator, solenoid valve, bubble counter and needle valve). The tank you can purchase or even rent usually from a welding supply or machine shop, new they run about $50 - 60 dollars.
To get it into your tank, how do you do it now? It might be good enough to use with a pressurized system.