I do advise getting some type of bubble counter - it doesn't matter what type, as you'll only be able to use your own system to compare it to.
A digital pH checker with probe would be useful, but an analog test of some sort would only be a very vague indicator. In addition, getting an accurate CO2 reading from the pH/KH chart is difficult, because it doesn't take into account anything you have in the tank that could be buffering either of those readings. Bascially, I'm saying that spending $10-15 for a drop checker and 4dKH solution will be the easiest way to go.
Additionally, I'd suggest a good check valve, or several cheap ones, all used together. You want to avoid water flowing back into the needle valve and solenoid at all costs.