Here is a way you can find how much that glass will cost you. I have the Starphire glass on my tank, and I would reccomend it if you can get it, its very nice.
Just use common sense. I would start by making a floor plan of your house. Detail where the structural supports are and what other furniture or applicances will be in the area. Typically along walls is the best place to put these things, but you have to accomodate for other heavy objects.
I am thinking that if you currently have a 210 gallon fish tank in your home, that a 12 foot 18 inch high tank wont be an issue. I did not read all of the pages but i think you mentioend that you currently have a 210/