+1 to what Diana said.
I run 30" Coralife T5NO fixtures on my 20Ls without a problem. Easy to use fiberglass window screen to cut the lighting levels down by roughly 40% if you find it necessary.
A T5NO fixture and a glass cover should set you back less than the hood you've listed.
About substrate... are you set on Flourite or gravel? Depending upon your budget, you could easily do a bag of ADA Aquasoil or Azoo Plant Grower Bed and have decent root ferts without having to add them for quite some time. If you're not worried about that sort of thing and are fine with adding root tabs, then you may want to consider Safe T Sorb instead of Flourite. It has the ability to absorb and slowly release nutrients, looks like Flourite and costs less than $5 for a 40lb bag (less than $10 shipped if you can't find it locally).