Secondcitycorals is steering you in the right direction, with good information.
My question though, what are you planning to use as a controller for the reactor? If you have an Apex already, the best method would be to purchase a PM1 module and PH probe for the reactor. Make sure to get the low range calibration fluid, and not the high range. If you do not have an Apex, Milwaukee makes a standalone controller that works well. Both of these systems will be able to control a electronic regulator if the ph goes out of range.
Now for a regulator, hands-down I would not get anything besides a carbon doser. I have had many different regulators over the years, and the carbon doser has been hands down, the best. Worth every penny, IMO.
Your reactor should come with a circulation pump, depending on what model you purchase. But they don’t typically come with a feed pump. There are many different feed pumps to choose from, but I would recommend a continuous duty pump like the Kamoer FX-STP Peristaltic Dosing Pump.
Next would be choosing your choice of media. A lot of people use ARM, but it’s just overpriced and repackaged dead coral skeleton. You can buy the same exact stuff in large bulk bags.
In addition to the media, I would also recommend using some type of magnesium media. I personally use zeovits, “zeomag”.
Other than this, you should be good to go. Depending on where you are placing your reactor, you might need some longer tubing.
