Props on not succumbing to light hype. See where that T5 gets you while you save money for CO2. Your plan to possibly add another is smart not only because those fixtures are cheap, you will have far more options using two strips rather than one, options for power, placement, spectrums, and it even sets you up with an easier resale (a nice medium powered, cheap and efficient light will fly out the window, plus you have two of them).
All those questions you were constantly asking have served you well, I think you'll do alright at this stuff as long as you listen to the right answers.
Remember that CO2 will benefit your tank even with low light. I wouldn't add a second fixture until CO2 is running steady for a while. DIY will suffice for this tank as long as you diffuse it properly and keep it steady, I would run two bottles with mix day staggered two weeks apart so you don't get a major drop in CO2 when the yeast quits working at full potential.