I am not really worried about the stand and the weight as I think it was built well, as I will explain below, but rather worried about the tank itself before it is built.
8mm glass usually has less distortion and not as green of a color, but one shop says it is ok, the other shop says it isn't.
One says it will need glass rods to support the corners the other says it won't.
One says that it will probably leak in a few years if I use 8mm glass, and the other say 10mm glass will make the tank too heavy and is pointless and 8mm is structurally strong enough to support all the outward force put on it by the water. As the weight of the tank is being supported by the stand and not by the tank itself (Unless it is only being braced at the corners and not on a flat surface - someting not being done!)
These are pictures of the back support which are under the top inside back of the stand.
There is a inner support which goes from top to bottom of the stand to further brace the weight from the top (middle board from above picture)
The front of the stand is supported by two layers of 18mm MDF, so 1 1/2 inches wide screwed together joining the sides, top, and front with 2 inch screws
The back braces, screwed to the sides will hold the sides together so now worries about the stand bending to one side or the other. And since the back braces are at the top and the bottom, each more than 1/3 of the total height of the stand it is well braced. (The opening is to allow heat to escape from the stand, make it a little lighter and give a place to grab when moving the stand) Plus I can use it to route the cords, filter pipes and CO2 if I don't wnat to drill holes in the sides.
Do you think this is not enough?
I have looked at the stands in the shops, and they have no inside back braces, the back is usually just 3 mm thick cheap plywood where mine is a solid back. They usually have an inner back support 1" by the lenght of the inside of the stand to support the whole weight. There is no braces at the bottom of the stand and the wood they use is a lot thinner.
The only place I think might ever have a problem would be the bottom of the stand where the sides join to it, but that would only be a problem if someone was standing on the inside of the stand and trying to lift up the top of the stand while still standing on it. (Unrealistic but it might happen) I will put two 2" by 2" boards to support that joint on each side.