I'm in the same situation you are, except i already got sump under way. But, you dont need tank drilled (dont think you can drilled tempered glass), you just need an overflow box. Check www.marinedepot.com and check under overflow boxes. You can buy different ones at different places, but they all work the same. You can even build one, if you want. Just ask dakar, i belive hes got a diagram or DIY plans on how to (think he's made a couple of them for himself actually). As far as size of sump.... like many other things in life.... bigger is better lol. I'd say go with a 50g if you can afford, give you a lot of space to work with inside sump and a larger refug for more live rock. Just my opinion.