It may bow a couple mm but nothing too drastic, it also depends on the quality of acrylic. The cell cast is the hardest so it shouldn't bow at all if that's the case although the price is pretty low for that type of acrylic. There is also continuous cast acrylic which is common for 3/8" thickness and is still good stuff but it's cheaper than cell cast and not quite as strong. Either way it's a great deal, I couldn't purchase the material for much less and if the manufacturer took the time to bend the corners it means they know what they are doing so the seams should be solid.
If you're OK with an acrylic tank and like the size and look of this one I would say go for it. You won't get a better deal and besides the overflow box being a little small it looks really good.
I wouldn't worry about the scratches, nanos are easy to clean carefully and you can always buff them out if you make a mistake. They don't scratch as easily as Myka mentioned, they won't scratch from sand blowing around
unless it's in a sandblaster