RE: 1/12 scale LED 3mm, 5mm, or 10mm
The body and the chassi will work fine together, so no worries! But to me it looks like the body comes with light buckets, or am i wrong? If it does, i guess they are maybe predrilled for a certain size of leds to fit, but don't quote me on this.
You can have brakelights, but without using one of these light controller-boxes of different makes on the market you have to do as i described above by using an extra servo connected to the same signal wire as the signal wire to the esc. Then the servo will move in one or another direction when you are braking pressing this switch to engage the brakelights. The only problem is that the brakelights will stay on as you are reversing the car aswell, this will maybe look a bit stupid.