You basically have 3 options.
The carbon doser is hands down the best and most accurate option, but it's probably unnecessary for a CaRX.
Other 2 are single and dual stage regulators. Dual stage are significantly better and when you see suggestions on building your own, this is typically what people are suggesting building. I've done them a few times using retired O2 or CO2 medical regulators, and it then just involves changing out the input and outputs and adding a bubble counter to match the standards used with CO2 delivery. Good quality brass connectors are expensive though, so this is a way to get a top quality regulator, but you're going to pay quite a bit to get it in tank-ready condition. The specific reason they are better is they offer a consistent flow rate no matter what the bottle pressure is. Single stage regs will have a variable output depending on the bottle pressure which becomes very apparent as the bottle gets closer to empty.
In SW, this isn't such a big issue, because you're adding CO2 very slowly and mostly confining it to your reactor, while just dripping reactor effluent into the display system. In FW planted tanks, the CO2 input can be very high, since the entire tank's PH is being lowered, not just the few gallons in a reactor. Without an electronic controller, single stage regulators are a disaster waiting to happen, and additionally will wear out a solenoid and even the wiring and house breaker quicker as the solenoid is relied on more to control CO2 instead of the bubble rate. Solenoids tend to draw a lot of power when switching so can trip GFCI's or even breakers.
As far as a budget setup, Milwaukee, Tunze, and a number of other single stage regs are all about the same in reliability. Many of the reviews are likely the solenoid and not the regulator, there's very little that can malfunction with a regulator. I found that I didn't need to run my CaRX off a PH controller, but I seriously eased into it. If you have an Apex with an extra PH port, you can automate the PH in the reactor this way. However, it can become a bit of a battle since you have to balance the flow rate as well as the CO2 rate. It's sometimes difficult to find the perfect amount, I would use a PH probe as sort of a last resort to shut off your CO2 if it gets way out of control. I wouldn't use it for CO2 switching to control ph for the same reasons listed above.