Well the main problem with building your own is the fact the glass prices are higher than what you would pay for the same size tank at a LFS until you get to the 200+ gallon range. I was quoted $300 for the glass needed for a 29 gallon tank and it only went up from there.
Thou a custom sized tank like you've described wouldn't be available at a LFS and would almost have to be hand built.
Most glass stores won't/can't ship the product (for obvious reasons) and I have yet to find one which lists the price on their website (usually states "call for prices"). Just look in the phone book for glass or mirror shops and they can usually give you a quote over the phone if you can provide the dimensions and thickness you require/desire.
One thought I had which might save a great deal of money would be to construct 3 of the sides, left, right, and bottom out of plywood with only the front being glass (or even the front and back are only glass parts). There are several write ups I've seen about 'plywood tanks' and from what I can remember they hold up the same or better than glass tanks. The plywood is simply covered (or more like soaked) in marine epoxy which makes it water proof and then simply have provisions for the glass front/back (usually involves 2 beads of silicon with a recess cut into the wood before the epoxy is done).
I would recommend 1/2" tempered glass as a minimum (tempered being stronger), only because of the distances you'll be spanning and the fact that someone could potentially set something heavy on top of the counter with the tank. If you did as described above (plywood tank) you could even get by with 2 or 3 separate "viewing" windows instead of one large one.
At any rate I think it's an awesome idea!
