I can chime in here a bit. having a calcium reactor has been an amazing addition to my 300! But when shopping around I strongly recommend against buying most every aquarium c02 regulator. Instead I suggest you get on ebay and pick up a stainless (brass works too but will corrode over time) dual STAGE regulator that is designed for corrosive gasses (others will work but the gaskets internally can fail over time)
A lot of regulators are marketed as dual GUAGE regulators and that is very different from dual stage. A lot of the horror stories from calcium reactors are because of the regulator being used. So heres the difference between the 2 and why its important; A single stage regulator maintains a maximum pressure for gas flow in a single step, but as your tank is used up the supplied pressure decreases which means you will have to tinker with your valve much more frequently. It also means as you reach the end of your tank and the pressure drops significantly it results in a lot of c02 dumping into your reactor (as in everything that's left in the tank) Where as on a dual stage regulator, this is done in 2 steps which results in an almost set it and forget it scenario. This is because in the first stage the pressure is set, and the 2nd stage is where the flow is set. The only variance ive seen in c02 flow is at the very end of your tank, and it can not release more gas than you have set, only less. This is where your ph controller (in your case I believe its an apex) will alert you that your ph has risen above your specified range inside the reactor. On my regulator I used the bubble counter, needle valve, and solenoid valve from a Milwaukee unit and the bare dual stage regulator I got on ebay. ( a good solenoid valve is very important as well)
Next is on to the reactor itself, I only have a single chamber reactor but I can definitely see the potential of a dual. With a 2nd chamber the ph of the effluent is increased a little more before being added to the tank which also makes the whole thing more efficient as well.
You will need a ph probe, Neptune has two different ones now and I have been using the double junction as its supposed to be more accurate over a longer period of time. Don't cheap out on the probe, as its the key to maintaining a good ph reading in your reactor, which makes the difference between success and failure.
Your reactor should come with a circulation pump so this shouldn't be a major concern other than making sure its not a cheapo pump that comes with it. My reactor came with a blue line pump.
Lastly you need a feed pump. A lot of people like and recommend using a peristaltic pump because of its consistency over time, however I went cheap on this one and got a cobalt aquatics (mj-900 equivalent) and a little plastic air line valve to restrict outcoming effluent.
setup seems pretty intimidating when your considering it all and especially looking at the hoses, and all the equipment, but its a lot simpler than what it appears. You can do a lot of things to adjust your output like changing the set ph in the reactor and adjusting the effluent flow output. Different brand medias start to dissolve at different ph values as well, so once you get things dialed in with one I suggest stick to that brand.
Eventually if the Neptune trident ever releases I plan to put my dosing pumps back in place and let them make small adjustments as needed while my reactor does all the big work.