The ladder diffuser came with the kit but he used a glass diffuser instead, he didnt run two diffusers. He did this for looks, but the one that came with the kit would have been fine.
The JBL profit 1 kit comes with a diffuser (they call it a reactor but its the same thing, just two names for the same bit of kit).
When you put the kit together, one end of the CO2 hose attaches to the regulator on the cylinder, the other attaches to the bottom of the diffuser. When it is running you use the needle value on the regulator to control to flow of the CO2 into the diffuser. The bubble rate is basically the rate a which the bubbles enter the diffuser. For instance I have mine at one bubble every 4 seconds. On larger tanks you need a higher rate say one bubble evey second.
I think you're getting to bogged down in trying to work it all out. The profit 1 kit has EVERYTHING you need to add CO2 to the tank. Its very easy to assemble and you'll quickly work out how to control the bubble rate. It really is very easy, trust me on that

get the kit and if you find you want to change bits, like getting a glass diffuser for looks sake, then get it then rather then trying to work it all out now and get the 'perfect' kit stright off. The expensive bits of these kits is the regulator and the cylinder. The rest can be changed relatively cheaply

(and that includes replacing things if you break them! Not that Ive done that of course!

)
Glad to hear you've got a maidenhead aquatics near by, I would phone and ask if they do CO2 refils, just to be on the safe side
Sam