I have built a lot of tanks and repaired a few. Repairing tanks is an absolute pain in the posterior and A lot of people (including me)will charge you heaps to try and avoid doing it. It is therefore often better to get one built as a second hand one can also be scratched or chipped (which considerably weakens it). I had one biult recently which was a bit fancy (a terrarium with vents and holes drilled for wiring) and I am sure the finished price was less than I could have bought the glass for.
The plywood floor to the bottom one is 15mm off the floor but supported by 15mm of polystyrene. I guess you could build a tank to go in there from glass or fibreglass.
They are the thumb grips on the glass sliding doors. Would you believe it costs as much to get the edges polished as it does to buy the glass. The wee ground out thumb grips that you see on the imported ones cost $70 for each one but the little shiny thingies (drill a hole and put in a plastic bit that looks like chrome) cost $8 each.
To grow plants realy well you need strong light (which you are aiming for) but you also need sufficient fertilizers and available carbon. Strong light by it self is not the complete answer.
That idea sounds pretty good. The only thing with mine was that the middle shelf (floor of top one) was only supprted by 20x20 battons back and ends so I was a bit worried how much weight it would carry. I would like to have added more supprt to the middle with a beam from front to back but couldn't because of the 4 ft flouro. Will try and wake up my technologically crippled mind and post a pic.
Similar to mine only I used glass sliding doors so they don't scratch so much and 90 x 30 frame around the front and rebated to take the plastic slides for the doors. I got one of those automatic flouro light controllers that produce dawn and dusk from that site that shall remain a nonny mouse. I put the tubes in the top front so they can't be seen. Be aware that they are sold as being able to take tubes UP TO 40 watts but are only good for tubes OF 40 watts or they will kill the tubes.
moist sphagnum moss can be OK as well, seed growing potting mix or fine peat. These work well with emersed plants but may be no good here as strong light, shallow water and nutrient are going to be loved by algae.
I have just finished one very similar that is 1600x600x1200 high and with glass sliding doors. I built it from 12mm ply screwed from the inside into 50x50mm cut into an L shape and around all the external corners. Well painted to seal the fumes from the ureaformaldehyde used in making the ply (it gives off formaldehyde when heated) The recess in the beading was 15mm so I added 15mm polystyrene to top and bottom. 250X250 S/S mesh vents at each end. A 4ft reptile flouro in each as well as a 60 watt ceramic heater and 100watt incandescent light bulb.
They work well. The only thing I think that would be better would be more support under the middle floor but it was tricky because of the 4 ft flouro. The bottom one is supported by the polystyrene.
I would get a stand built from steel box section and designed to spread the load as evenly as possible over the floor. With the weight of the glass and the head of water there will be massive forces on the floor.