You might want to slow down and take another breather. The price is more than just too high. My 96x42x36H tank, 3/4" starfire glass 4 sides made by AGE in Garland, Texas, with superb construction and my custom built TIG welded stainless stand were a fraction of that cost. See my post above #38. My stand is 8 legged fully rimmed and cross braced, top and bottom, 2" square tubing, stainless steel. It was designed for over twice the weight it will carry and some pretty strong lateral dynamic loads, over-designed for sure.
You should, indeed, be impressed by a tank builder whose tanks have never leaked. But understand that the best tank builders' tanks don't leak. Don't get carried away on this one point to where you pay through the nose for the extra security that is not that extra. Talk to tank manufacturers about what their leak history is, but also talk to them about what they will do if a leak does occur. That is far more important than finding out you have the first tank that the builder ever had spring a leak. What is the tank builder's commitment to the future of his tanks, not just his bragging points about his past. Then get it in writing!
If I ever do another large display tank I would increase the width (front-to-back) before I would increase the depth of water; mine is 36" high. The top of my tank is at 6' 2" from the floor under a 9 ft ceiling which is just about minimum for mounting lights and for working on or in the tank. I am a big guy. I built a light frame suspension system with a hoist to move the lights up for when I have to access the tank interior. I have a decent equipment room for all of my support equipment so I do not have to work in my tank except for unusual situations, which is preferably never. I work in my sump and other support tanks.
Were I to increase the depth, I would not even consider going above 42". That drives costs through through the roof for the tank itself. That cost only starts with the the thickness of glass required to hold the pressure and builds up from there. It there is ever to be any reefscaping with corals, lighting can move from a difficult proposition to a nightmare, above 36-42" tall tank. If it will always be a fish-only tank forever lighting is not critical.
In your earlier posts you talked about the access above the tank as being pretty cramped. Extra tank height goes against this unless you mount the tank a foot above the floor. You absolutely need an equipment room.
Have you planned a quarantine tank arrangement? ... and maybe even hospital tank(s). They are cheap, more necessary and extra valuable when you are operating a large display tank. With large display tanks, disease breakouts are extremely difficult to fight in-situ when you do not have hospital tanks... again equipment room. Also, UV sterilization isn't cheap if you do it right, but it can go a long way to preventing breakouts.
Preventing and fighting breakouts does not have to be about saving a fish or a coral, it is about saving your tank community.
IMHO, going over 42" tank height is very problematic on several counts. Putting a small amount of extra cost into increasing the extra width will provide a far more aesthetically valuable increase in the visual impact of a tank than height above 36" or 42".
Please back off of this overly expensive too-tall tank for a lesser expensive alternative that will probably give plenty of viewing impact, probably even more visual impact if you drop the tank height to 36-42" and increase width to 42 or 48".
Conserve funds now. You have a long way to go in equipment and tasks and you are looking at 40% of your budget gone before you've even started tackling the myriad expenses for top-end equipment and setting up a well electrified and equipped space for support equipment. Complete this task for your friend for $75K and be greatly appreciated and praised, $95K and still be a friend who did a great job to help a friend, $110K and you are just another contractor who couldn't control costs.
As a previous poster said $100K is in his rear view mirror. I can relate to that and I didn't spend anywhere near 40% of my budget on just the tank and stand. My point is that expenses "post tank and stand" can be surprisingly high with large capacity systems. Don't paint yourself into a corner early on. On top of that you have to keep some reserve. There will be unexpected stuff happen and it won't be free.
One other thing and this is speculative on my part. Your wealthy "client" friend is probably capable of changing his mind abruptly when it is necessary or convenient for him. He may like the tank a lot and grow to like the beauty it provides his rooms more than he even expects to. That may draw him to reefscaping with corals. Don't lock him out of that option. Lighting this very tall tank is a giant problem. The tank is a room divider. Seeing through a tank into the other room is distracting and not as attractive as looking at a reef, deep into the tank, set back from the glass with a good amount of foreground. Having similar a similarly spacious view from the other room also will really be an extra touch of class. A reef that blocks the see-through view may be a visually powerful feature for the room-division purpose.
I wish you success in learning throughout the journey this tank build will turn out to be. Most of all I wish you success in having your friend be overwhelmed with the wonderful tank he has as his new room divider done by a friend who did not become just another contractor who couldn't control costs.
And thank you for sharing your journey so openly with all of us.
cheap for powder coated stand made from 4x4.. 
