I'm planning and budgeting a 100L planted tank

My LFS does not carry undergravel heaters. He says "they are not being
produced any more." Any reasons?

Substrate: Thinking of mixing 1mm quartz sand with SeaChem Fluorite to
reduce cost of substrate (50:50 or 2:1 ratio). Problems, thoughts? Will
proper feeding compensate. How about creating a plenum using egg crate
instead of using the sand to increase the depth of the substrate?

Thanks