Most of this is rubbish. You do not NEED a CO2 controller and you do not NEED the electronic potentiometer. Thousands and thousands of hobbyists have had much, much sucess without both of these items. Get one if you WANT and if you have the money. Make your desicions wisely.These I think are the best of the best but some may debate. You will not go wrong with these:
Ok from someone who has tried a few of those needle valve/bubble counter/check valve "Rube Golberg" things, here is really the only regulator I would buy because it is the only regulator designed specifically for the aquarium plant grower:
It is expensive but since your going for a 125gal tank you are not thinking cheap. This regulator has everything built into it including a bubble counter, check valve, and solenoid. You control the flow with an extremely accurate electronic potentiometer, not a needle valve.
I would get 2 x 5# CO2 tanks each one should last a couple of months at least; so, you always have one ready when you need it. A 5 pounder if it goes will not produce excessive CO2 levels in your room (Unless you have a very small bedroom).
As for controller (yes you need one) this is super, I have one and it controls CO2 +/- 2 ppm the UltraLife MC101:
You might get it cheaper at you Local Fish Store so ask them.