1. change your substrate as few times as possible! lol. if you are planning a substrate like eco-complete, i would personally wait until i was ready to add plants as well (or soon there-after). i'm not sure if it will cause algae issues or lose the nutrients (although i will have to assume that it will not) but always better safe than sorry.
2. light is loosely measured for an aquarium in wpg
(watts per gallon). for low-light plants you need ~1-2wpg. there is also something called the k
rating, which indicates the color spectrum. plants can only use light in the spectrum from about 6000k-10,000k (6500k being the most common spectrum for plant bulbs). if you are only planning on low-light low-maintenance plants (like java fern, anubias, crypts, etc) then your flourescent fixture should do the trick. before i upgraded, i used the fixture that came with my tank (with 18" 15w flourescent bulb) and just went to walmart and picked up a "daylight" bulb (6500k). i had anubias barteri, crypt wendtii, and java fern that all did beautifully.
3. for low-light set ups you do not need a CO2
system. however, flourish excel can be helpful.
) most crypts do great in low light, and just about any anubias.