So you are looking at a custom starphire 180+ gallon tank? Do you want it to be rimless? Honestly, you are looking at $3-4000 for that size tank, if not more!
I had Miracles up in Canada quote me price on a 36x24x20 rimless tank and it was about $1300 shipped. Your tank is over 2x as big and will require thicker glass with the added height.
I would never DIY that large of a tank and risk my house with that much water damage but that is just me.
I read an article a few years back that said it costs about $50/gallon to buy and stock a tank. I always thought that was extremely high but then I did some calculations and it was right on if not a little low even. Buying used can certainly help keep prices down but I would still plan on at least $50/gallon for a tank.
Heck, on a 180g tank the light would cost me around $1000 alone!