the larger you get, the more quality becomes important
400g of water is a massive disaster if something goes wrong.
what I would do is this: try and save up the cash and have an experienced tank builder build your tank.
building your own is certainly do-able, but you don't want your first tank that you build to be one that big. if you make a mistake because of inexperience/bad advice followed etc etc, well, you have a massive flood..
if you want to build your own 400g, perhaps try building a small tank first? as a 'tester' if you will? then move up to the monster.
After about 300 gallons its cheaper to DIY. If you don't have any woodworking/building experience, building your own can be pretty daunting. There is a TON of wisdom and knowledge on the DIY section of this site, read it, learn, and give it a try.