I've made my suggestions on what I like, so perhaps I can point to some things that I don't like as well, without seeming to just be a nag? I know there is a fine line between info and nagging but when we are new I find it worthwhile to get down to the details on things that may not be obvious at first. My suggestions do not mean that other things will not work but I would like to point new folks to some of the details as those small points are often the ones which do give us the most trouble with the CO2.
The reg body itself is rarely the thing that fails us first as it is a pretty simple thing when we get down to looking at single stage regs. Basic design is that there is a diaphragm that finds a balance between the gas pressure from the CO2 tank and a spring which we adjust with some type of screw to set the outgoing pressure. Pretty simple and not much goes wrong if things are not truly abused.
But the small parts? They can be a real bad deal so we need to look closely at those. The "gate" (solenoid?) that turns the CO2 on/off is one that can gives lots of heartburn and fail way too soon if it is not a good one. There is the basic "black box" that is often found on ready made units but they can have some real problems as they use a standard method. There is a small pellet of metal that slides inside a small tube when the electro- magnet is powered. To be tight enough to hold gas, that fit has to be tight and that can become a problem due to the long time period we power the solenoid as it can get hot enough to dry any lube and then it may make the parts swell and they stick, either open or closed. The amount of power used is an indicator of how much heat will be generated much like a 60 watt bulb gives more heat than a 25 watt. I think there is kind of a general idea that 7 watts may be too much while 4 watt is workable. I like the Clippard Mouse series as it only uses less than a watt (.67) to move a tiny metal "spider" rather than sliding a pellet. Something like a reed valve, if you are familiar with small engine work. They should last nearly forever in our use.
Lots of info from Clippard here:
The needle valve is the next point where we may need to go for more quality. Several things are involved in making truly good valve for our use. Along finely tapered needle moved in a precision hole with precise threads will give a really fine adjustment while a short stubby needle made from cheaper metal and course threads is kind of like sticking your thumb in a hole to control flow. Hard to set it to 3 bubbles per second versus four! And the cheaper ones tend to move as the gas cuts on/off, so that it requires more fussing to keep it steady. The better quality valves have "drag" built in so keep the setting from drifting.
Not all ready made are bad and many find them good enough but just some points to consider.
Buying a CO2 setup is expensive but it can make it even more expensive to buy a cheap one and find you want to replace it within a short time as it is driving you crazy!!
No perfect answers but some points to look over when shopping! It's a trek, for sure.