Hi,
For substrate, we have a really wide selection, so I would browse the substrate section. Everyone tends to have different tastes, but I personally think Tropic Eden Mini flakes is the coolest stuff around.
On Rock, Timpora is the best quality rock right now. So it would be a great choice. Tonga is very good too, although larger pieces, but for a 75gal, you probably want a decent amount of bigger pieces.
I think 75lbs is just right, you can add more for personal taste/looks, but 75lbs will be enough for biological support.