[Gomer]

There are all sorts of substrates that work well, but I personally like Eco complete for its dark color and not needing to rinse it.

Not sure of a 100g, but a 90 Gallon 48x18x24 will need 7bags of eco complete

A 120 Gallon 48x24x24 will need 10 bags

This assumes ~3" depth

 

[Gomer]

Just use eco by itself. If you have mulm from your previous tank, feel free to use some of that to kick start the tank.

Eco complete effectively lasts forever.
I'd say the same with flourite and onyx as well.

Not sure how long florabase or shultz will last before breaking down physically... if you are thinking really long term.