Most reactors are all pretty similar - they recirculate water, let in tank water, let in co2 and let out effluent. They are simple beats. Built quality matters. That being said, there are literally a dozen brands that are totally fine - I use Korallin, but I would also use GEO, LifeReef, AquaMaxx, Knopp and many others. I like reverse flow, but with larger media not clogging as much, it is not as big of a deal as it used to be.
You can spend a fortune on regulators and stuff, but if I were starting over, I would just buy a Tunze and call it a day. I have a very expensive dual stage stainless Victor with oil filled gauges and a custom needle valve - the dude that I got it from spent like $750 on it and while it is nice, it does not do any better of a job than the Tunze that I am using, or a m3 regulator. The expensive ones are easier to fine tune, but you can fine tune a cheaper one just as well. You can always add a super-precision needle valve to a "normal" regulator.
I would skip the ph probes and learn how to tune this thing by hand - it is easy, more reliable and will save you trouble down the road. Take the time and learn... it seems like a lot at first, but with just a bit of effort, you will "get it" all of a sudden. Once you learn, if you want to monitor with a pH probe, then cool, but you will be better at this if you know how to do it yourself.
The only media that you can get right now is calcite or dolomite - if you have a LFS that has some Reborn or Natural ARM (old stock), then grab it, but it is hard to come by. The current incarnation of ARM is crushed calcite - get the medium size.
I feed with a cheap pump like a Quietone 1200 or Maxijet - they seem to last 3-4 years, but they are like $20 to replace so no big deal. I also use a mainline T on one system. You can get an expensive dosing pump (Kaomer, for example), but make sure it is continuous duty. Some reactors will feed themselves, so check this out first. They all work plenty well. DO NOT run a calcium reactor off of a manifold.
If you tune your reactor well, a second chamber will do nothing. If you plan on learning how to tune, then no need in buying one, but if you are sure that you want to use a pH monitor and probes, then get one for.
If it were me, I would try and find a used high quality reactor - Geo, Korallin, etc. Buy a new Tunze Regulator. Get a Mainline T to feed. Get a flow control pinch to control the output. Find a 20lb bottle locally - 5 or 10lb are fine, just need refilled more. Get some co2 safe tubing. Order some ARM. Get a Salifert Alk test kit - cheapest and easiest to check effluent. You should be able to do all of this for a few hundred bucks... or you could max out, and go over, that whole stimulus check if you really wanted to.
