Hi
You'd need the regulator, a diffusor of some sort, plus CO2 proof hose. Additionally you might like to consider a solenoid to connect to a standard timer to shut the CO2 off at night. Depending on how much CO2 you're running into the tank, a 500g bottle would last 2-4 months. It really depends how much you crank in there-I'm a bubble per second plus man myself, so I can get through it pretty quick. The main thing is to ensure somewhere nearby can refill the gas for you. If you check aquaessentials, Richard is doing a complete basic JBL set for £119 I think. This will have all you need for a basic set-up. Given that a decent regulator will set you back about £80, you might find it better value to go for the basic JBL kit over trying to put one together from the component parts. I keep 2 bottles per CO2 set-up so as I always have a spare. After you've refilled a few times, refillable systems pay back over disposable units as well. You won't look back once you've gone pressurised. Hope this helps.
Nick