I think my 40 breeder had about 80ish pounds of eco complete. Maybe 90. So, 4-5 bags should suffice? But, we actually have a Substrate Calculator
built into this website. You might want to check that out for a better idea.
That said, I would totally vote for skipping paying a fortune for substrate, and instead going the dirt/pool filter sand/blasting sand cap method instead. For the price of one bag of eco, you can do your entire tank, and long term it will offer far more nutrients then eco complete.
HC won't grow in a lowlight tank. That much I know. I've never had luck with marsilea either, but I think that's just me.
You might want to check out fleabay for cheap lighting. 3 foot fixtures tend to be on the rarer side, and thus a little more expensive.
If you stick with eco complete, yeah, throw root tabs down. It's certainly not going to hurt anything. Dosing florish or excel on a tank that size is going to cost ALOT though. If you really want to dose, or incorporate co2, you'd be better off buying a regulator and co2 tank, and dry ferts from one of our vendors and going about it that way. Personally though, I'm not sure that it's necessary.