Buy a CarbonDoser.
Having built a few nice Dual-Stage regulators myself now, and also having re-built a broken CarbonDoser, the CarbonDoser is just easier. If you're DIY inclined, and like to off the deep end, then by all means, build a dual-stage CarbonDoser with the same high-cycle valve that CD uses, but if you're not DIY inclined, and would rather not be messing around (not to mention sourcing the right parts, making sure the parts you get work well, etc.), I would just stick with tried and true CarbonDoser.
I'm really frustrated that the $100 needle valve that I got for $50 used
might cause some inconsistency in bubble rate because it could have a bent needle valve (?!?!). I have it dialed in rock-solid now, but still, it was frustrating in the beginning to diagnose and get dialed in. With CarbonDoser, it's set it, test, and forget it. Minor tweak, simple turn of the knob, and you're done. No worries that parts could be malfunctioning or other frustrations.
Also, building one from used parts won't be cheap, it won't be easy, and you'll always be second-guessing the quality of the used parts, unless you're an expert at gas regulation, which you aren't, because you're asking us to begin with
