W.r.t. a spindle enable pin:
The dewalt is 600W, which is 5A at 120VAC. That’s too close for my comfort. There’s also something about it being an inductive load… There’s some kind of diode you need, I don’t remember right now.
You can easily control it with one of the digital pins. Even the pwm one for the cooling fan would work, but it is 12V, so you either need a 12V tolerant relay board, or you should reduce the voltage with a divider. Also,if you are using pwm, you’ll need to only use 0 or 255. Some low pass filtering might be necessary, but I would try it first. You will not be able to control the speed with a relay.
I second the idea of building it into a box, AC will kill. No joke. There are also boxes that have an AC plug, and take arduino voltages as input. They are about $40, IIRC.