The 13 watt bulb won't be sufficient to grow much more than Anubias, Ferns and Mosses. The retrofit kit is probably a good idea, especially since you already have the Seachem ferts on the way. I'm assuming that Seachem Excel is among them for use as a carbon source?I'm thinking of upgrading the stock light (13W, 5500K) with a Current Model 1611 retro (32W, 6700K/10000K). However, before I do so I wanted to get an understanding of the pros and cons for each lighting system, i.e., what type of maintenance, fertilizing, CO2, etc. would be needed and the type of plants that can be grown. The type of plants is open at this time. I have a list of "low-light" plants to choose from. There are many. I'm not particularly interesting in a "high" tech tank but at the same time I'd like good, healthy plants and fish.
Seachem's substrates are nice, but they won't provide all of the nutrients you'll need for a successful planted tank. The crossover point for CO2 or not is more by feel and experience than anything else. I can tell you that any tank will benefit greatly from the addition of CO2, even a low-tech tank. In my opinion it's the single most effective "updgrade" you can make to any planted tank. If you want to start out small you can use a DIY CO2 setup using yeast, sugar and water.I have Flourite Black Sand on order. Is this sufficient or should I supplement with an plant soil? I've read that natural tanks use a plant soil for the nutrients which reduces/eliminates the need for CO2 and fertilizers. This sounds interesting in that my son (and me) will have to worry less about the biology/chemistry and enjoy the tank more. However, I also understand that as the lighting wattage goes up, the need for CO2 and fertilizers increases. I do not know that crossover point.
Natural tanks can be a lot of fun too. I'm running most of my tanks using a mineralized topsoil substrate capped with various different gravels and sands. I wrote an article on this method here.
Whichever route you decide to go you'll need patience most of all. You'll make mistakes along the way and sooner or later the dreaded algae will appear. Just take a deep breath and try to learn from it and you'll advance quicker than you think.