You are ready for a CA Reactor when you feel you are spending to much on dosing supplies. How much is to much is up to you.
I run a Precision Marine 6 X 24 Calcium Reactor W/Eheim Pump and feed from a line from my return pump. I have a carbondoser electronic co2 regulator and a 20 or 25 pound co2 tank, I can not remember now what size it is. The bigger the tank the less you have to get it refilled. I have no controller, I just go with my ALK tests to make my adjustments.
Been running it this way since July 2004, The setup works other than the Eheim pump is glued to the reactor. To me this was a mistake Precision Marine made in the design. I have no way to rep[lace it other than break it off, I have found several videos on how to get it off. The pump is just now seeming like it needs some work done on it.
I am currently waiting on a new reactor to be built. I ordered an 8" Automated Calcium Reactor from Aquarium Engineering.