12v drills are a great drivetrain for a first robot, I wouldn't so much recommend overvolting them just because it can cause them to break quicker (especially if they don't have all metal gears) however you can get a bit more power out of them with that.

If you wanted to get fancier and a little easier to mount in a machine assuming you have a larger budget (Drill motors need modification, there's a few guides out there) there are ready to run gearboxes out there with easier mounting points etc, Gimsons used to offer the GR01/02 range but have since been discontinued though you may find a pair for sale on the forum from time to time. I'm led to believe both Shockbots and Ranglebox are going to be offering gearmotors in that sort of light too

There's a number of speed control options available for those too, the Botbitz TZ85s are pretty commonplace these days in machines, handle pretty much every motor you could throw at them on a featherweight scale.