Not outside the skills of most people. Do it. Make sure it's not tempered first.

The glass is quite thick on a tank that size, so your risk of cracking is fairly low. Use hardly any pressure, and keep lots of cool water flowing over your cut. Get your holesaws from glass-holes.com. The thrill of success is exhilarating. Dooo eeet!
If you fark it up and break the back, all hope is not lost. You can remove the broken pane, and have a glass shop make you a new one. You can have them drill the holes if you like. Silicone it in place and you're done.
Of course, if you're already looking at a budget of $500, consider having a tank custom built - you can have them drill your holes for you, put in eurobracing or go rimless, and otherwise get a much better tank for your money. It costs nothing to get a few quotes. Word of advice, avoid glasscages. Search for them on NR and you'll find out why. There's a reason they're dirt cheap.