IMHO don't go over 3 wpg for lighting. You should be able to grow most anything you want at that light level in that size tank.
You really can't do a good planted tank "on the cheap". Planted tanks are expensive. But you do already have most of the equipment. So that does help a tremendous amount.
I would either go with Flourite or Eco-Complete as the substrate. If you put sand over the Flourite the Flourite is going to end up on top unless the grain size of the two substrates is similar. I have mixed Flourite with fine gravel/coarse sand in a 50/50 mix based on depth and had very good luck with it. But I was careful to keep the grain sizes similar. In another tank which started out life as a fish tank I mixed in the Flourite with the existing small LFS rock. Well the Flourite is now a base layer and the Roan River gravel is all I see. And it's a pain trying to plant new plants is the seemingly coarse substrate. The larger substrate is always going to end up on top.
IMHO there are no good methods for automating the fertilization process. You can add nutrients on a nilly willy basis without causing problems. By automating the process you will either end up with too much or too little of any or all of your nutrients. A worst case scenario would be a 100 gallon algae display tank.
When filling my tanks via the Python process I treat the entire tank volume with Seachem Prime just as the directions on the bottle state. As for tap water chemistry.... I mix up a couple of gallons of very hard water and slowly pour it into the tank while it's filling.