I was in your exact same situation about a year ago. I wanted to do an overflow box, but in the end, I bought a gl*******s.com kit and drilled the overflow. Long story short, that tank leaked and I now have both the overflow and the return lines drilled. From someone who has personally tried not drilling, it's way better to drill and not nearly as scary as it seems. Gl*******s gives you awesome instructions. As for the sump itself, I spent like thirty bucks on glass and cut it myself. I now have a chamber for my protein skimmer. Water overflows the front of that glass into a little section for growing macro, and over the side to the baffles, through the baffles to the pump and back up to the tank. My siliconing is a little messy, but it was my first try. Overall, it works, and I'm very happy with it. As for sump size, you are limited by what you can fit in your stand, but the bigger the better. I have a 29 because I built a stand specifically for a 55 display and 29 sump.
As for tempered vs not, there are methods you can google to tell the difference just by looking at the glass. It is highly unlikely that yours is tempered if it is less than ten years old. Tempered glass costs more to make, and as glass overall improved, the necessity to use tempered side panels disappeared. Worse comes to worse, you drain the tank and just tilt it to look at the sticker. I'm 18 and have drained tanks probably a dozen times. It's not as much work as it looks like.
Good luck, and I hope I helped.