Nah, sumps are easy. Water comes in on one end and get returned to the tank on the other end. In between it runs past and through heaters, skimmers, macro algae, filter socks, carbon etc. It is really a great way to keep all your mechanical filtration below the tank and out of sight instead of hanging it off the back or in the display. Here is a great site that explains it http://www.melevsreef.com/allmysumps.html">http://www.melevsreef.com/allmysumps.html</a>.
You are correct that you cannot drill tempered glass. However, most tanks only have tempered glass on the bottom and not on the side panes where you will be drilling. Do you know who manufactured the tank? If so you can usually contact them and they can tell you for sure. My guess is, and it is only that, your tank will be able to be drilled. If you decide to go with the hang on overflow box that will be fine. I have had them before many years ago and did not like them. If you have a power failure and you are not around to restart it you will get a floor full of water. There is a way to modify them with a small pump so that they will restart if you are not there but I will leave that explanation for somebody else.
Lighting is probably one of the largest expenses you will have. So many people want to go cheap with the lighting and then wonder why nothing grows in their tank. Another thing to think of is what you want to eventually have in your tank? Do you just want mushrooms or do you want to do the hard stony corals eventually? Go ahead and spend the money now for a lighting system that will support whatever your future goals are so that you dont have to buy twice. If you were to put another 6 bulb T-5 on that tank you could put anything you wanted to in there. A cost effective alternative will be to buy used when you can. Often times people will upgrade their lights from a T-5 to LEDs and will sell the old fixture for a fraction of the price of a new one. You can really get some great bargains. Keep an eye out on the "Selling and Trading" forum here.