In order to connect the regulator to a paintball tank, you will need an adapter.
This will allow you to connect the paintball tank male threaded fitting to the "CGA320" threaded fitting on the regulator (CGA320 is the standard American regulator/full size CO2 tank thread size).
A dual-gauge regulator will show you the pressure remaining in the paintball tank, and allow you to set the pressure coming out of the regulator.
You want to attach the needle valve to the output of the regulator. The regulator adjusts the pressure
output, and the needle valve adjusts the flow
What is the current setup you have with the DIY CO2? You can likely use a lot of the same tubing/etc.
You then attach the tubing to the output of the needle valve. It would be nice to have a bubble counter attached to this tubing (cheap online, or DIY) and then output in the tank either to the filter/powerhead or a diffuser.
A check valve is important to have in the line as well, to prevent water backflowing into the regulator and/or CO2 tank.
There are actually products out there that incorporate a diffuser, bubble counter, and check valve all in one.
In order to determine the amount of CO2 actually being used
by your plants, you can install a drop checker. It essentially is a liquid reagent pH test that is installed inside the tank using a standard KH value for the test solution. This liquid constantly monitors the pH based on the CO2 concentrations in the tank water. Since it is using a standard KH concentration, you can calculate the CO2 concentration dissolved in the tank water.
Another piece that can make your life easier is a solenoid valve - it operates on electricity and, when paired with a timer, will allow you to set WHEN the CO2 is injected ("daylight hours") and when it is stopped ("night hours").
There are "beginner" all-in-one kits out there that include a regulator, needle valve, and solenoid all in one. Some include a bubble counter as well. They're a pretty good deal, but you could get something of higher quality by piecing together your own with high quality components. Though the regulator/valve/solenoid kits are an attractive option!