I may be able to help a little with the substrate.
If your going to plant carnivourous plants into the substrate, put a very small layer (maybe 1/16-1/8") of peat down, then put the substate on the top. It will help out of the CPs alot, PH and such. With CPs in there you will want to watch your fertilizing or you will kill off the CPs. The Utric's I have growing in the fertiziled mud are growing 1/10 the speed of anything else and the leaf sizes are 1/20. So tiny!
I've used prima-agra with good results, doesn't look that natural thou, but from a functional point of view it does the job. Another problem with this stuff, its round
. So you won't have a flat or sloping area. Although you could cover the prima-agra with a sheet of Epi-web, that would look alot better and give you a slope or flat area....must remember this when I order from First Rays next......Wonder if they would cut me an odd 5 sided piece....
I've also used a pond mud I picked up in Japan some years back. It works perfectly in every aspect. So you might want to check out pond muds/soils.
If you have some extra money there is always ADA Aquasoil, its pricey but works wonders, and won't degrade. I've had nothing but good results with this stuff too.
Don't forget to check out the planted aquarium forums for substate idea's, anything they are using should work well for what you want. After all its intended to be submerged.
I would stay away from any substrate that will decay fast, like bark and such, the bottom will stay wet almost constantly and thus decay very fast.
I've had mixed results with using Epi-web as a background, it is clearly not its intended use. Epi-web really needs to get a dip in water to shine, and even with a pump pushing water into the top, the results are still meh. It just does not work its magic unless its submerged. Stuff is growing into it but the glass is getting covered faster.
On the live sphagnum....it looks nice and alot of people say good things about it, but it grows fast in proper conditions and will cover your plants if you let it. I've had it cover african violets before.....
Three last random thoughts, lights are cheaper per square inch when purchased in 48" strips, T5HO lights are your friend, and don't forget about emersed aquatic plants!