If you like the look of onyx sand, go with that, but I'm leary of two sand colors and the mixing issue.

That is a temporary display, it's fine if you do not mind and know that going into to it.
2 years down the road, it can look tacky having them mixed.

I've done a few such tanks, I like them, they look nice etc, but I know it's only a matter of time and I'll mix them, even with a good rock hedge etc and wood, but.......
one good thing, is the front decorative sand is cheap, where as the ADA AS or Onyx sand is not.

You might also try the ADA soil, cost about the same as Onyx, bit nicer color.
Then you just add the old "dirt" from any tank you have and add that.
Ought to work well.

I'd keep the light in the front for the fish(6x 39 w T5's) pointed back, towards the rear of the tank and the plant beds.

The fish will look better than only having light from the top, it'll hit their sides instead. This will also direct the light away from the front of the glass.

So yes, 4 or 6 bulbs would work fine. Realize T5's with a reflector are about 2x as bright as a T12.

So that's a lot more light than you think.
But it is more, you can reduce it and use 4x 39w also.
That way you have some options also.

Canisters allow for a few different flows, have decent steady flow etc, but you can use a sump and filter bag, sponges etc. The lok tubing that flexes and twist arund to point the return water any number of ways, DIY spray bars etc can be made for this type of filter as well. Those are nice and what I tend to use personally on large tanks. Tanks under 60 gal I use canisters, they are easy to use and quiet which is a huge issue for me personally. Some like 2 canisters for larger tanks and that works well. The sumps I make are a bit more complex but iof you are well versed with them and like, then by all means, use those.

Up to you there, just an idea, as long as you have 600-1200gph and good particulate removal, you are in business.

Regards,
Tom Barr