essentially its easy, the hard part it getting the matterial. i re seal tanks all the time, thats easy but u'd have to have glass custom cut and the get edging or the top and bottom edges. in the long run buying a tank is quicker and more cost effective.
i was thinking about doing the same thing awhile back ... i wold say you might be better off buying a used tank localy. im shure you dont want X-gallons of water on your floor becuse your tankbusted a seam due to it not being strait and level! if you do build a tank post about it and let everyone know how it goes. from what ive herd you haveto use the most level suface you can while building it to help insure you get a good strait tank.
good luck on the tank if you decide to build one.
ADDed: if you look at older tanks they have strips of metal edging over all the seams and along the top endge. somthing like that wold add strength. ive seen things that wold work for that all around. IMO if you find some kind of edging that wold work all you shold need to do is use the same silicon you use to seal the glass to glue the edging on.