I'd go with the T5's and mainly only in the rear. Leave the front with less light, anywhere there are not plants.

I'd use perhaps 4x 39w at about 4" above the tank top.
No, you do not need onyx sand.
A nice small 2-3mm white sand would look nice.

You may add a mix of 1/3 garden soil(soaked for 2-3 weeks in a shallow pan outside in water or boiled for 10 minutes) and 2 parts sand. Mix this good and add about 2" of this to the bottom of the tankl, add about 1.5-2" of the sand on top of this.

Plant choice is good but I'd highly suggest adding lots of plants from day one.

That planted section will need to be really packed well.
Add Zeolite and activated carbon in the filter also, use a canister filter( 2x 300gph).

I think a nice idea, especially give the design, is to put floating water sprite in the tank and direct the spray bars to corral the water sprite towards the front where the open area is to prevent algae and to block the light. Water sprite has access to CO2 and light there and it can look nice if you use the current well.

Salvinia and other floating weeds may be used also, but water sprite is easy to grow both floating and submersed.

Regards,
Tom Barr