Welcome to the adiction. Beware you will forever be spending a little here and there which can add up to hundreds of dollors. Starting with a 20 gallon is the easiest, for since it is not very deep it is easier to get the right light for it.
Yep, I am from AA!
What is AA?
For lighting I am thinking about Ecoexotic's Planted Tank
I would like to have a tank in which there is no CO2 supplementation or the addition of fertilizers. Would the Eco-Complete and PWCs be enough?
What is PWCs?
Checking out Ecooexotic's light and found it is made for reef tanks thus it probably is to bright and won't have the spectrums that the plants need.
Wheather you need ferts depends on what plants you decide on, the light, and the minerals in your water. For example ferns usually don't need ferts. In the beginning my ferns did great without ferts. Now I have to add ferts for them, for this year they are melting. I found a cheap all in 1 at AAA (atlanta aquarium association) by select aquatics
To keep it simple low light or light with programming is advisable. Good lights are Coralife T5NO (aqueon in Pet Smart) T8s, 2 clamp-style work lights with daylight CFLs and Led Flood lights. A programable light I am thinking of getting next year is Current Satellite Plus. Lights are the engine. To much and you need co2.
Plants that I will be having in a non 20g non Co2 are: moss, ferns, crypts, hair grass, pennywort, ferns, Milfoil, and Sagittaria subulata. There are a few more that don't need Co2. Check out plant finder
Substrates peater out in a year. Thus I think it wastefull to spend a lot of money on substrate. I have found that Scott's top soil with KNO3 (potassium nitrate from spectricide stump remover.) and special kitty litter works. Then top it with river sand from a landscaper. Scott's top soil has a lot of peat in it which adds Co2 to the substrate. The KNO3 is added for found plants love it. The kitty litter helps keep the dirt from compacting. Scott's top soil has to be sifted for it has a lot of sticks in it. After a year or when the plants, expecially crypts, get stunted you add root tabs. There are diy root tabs, seachem, and API root tabs, (which are cheapest at LNT.com)
I should mention the plants I have been thinking about are: Bacopa carollniana, Telanthera, Cryptocoryne wendtii, Green Horemanii Sword, and Dwarf Hairgrass.
Telanthera and Bacopa carollniana will need Co2 in the water. Green Horemanii Sword will probably get to big for the 20g