The "external mic"

assuming your music is loud - is only part of the equation. You also need manual audio gain control. Without that, there is no mic that can help the camcorder deal with LOUD audi being recorded as a muddy mess with static.

Assuming you want to press record and let it go for a while, a dSLR is a poor choice. Usually, dSLR audio gain control is poor - and they are known to overheat during prolonged (longer than 15 minutes) of video or "live view". dSLRs are designed to capture stills - and video + audio are secondary "convenience features". This does not mean they cannot capture good video - they can. But they cannot be treated like a camcorder. Once in overheat mode, they take a long time to cool. And they have file size and record time limitations that camcorders do not have.

My preference is stereo audio - so the camera/camcorder needs to have either built-in stereo mics (common to camcorders), and perhaps a stereo audio input - and manual audio gain control.

The Canon HF R series has very rudimentary "normal" and "attenuator" (for LOUD AUDIO) while the HF M series has about 12 steps in manual audio gain control. Both series also have 1/8" (3.5mm) stereo audio inputs. Audio Technica makes decent stereo mics... so do RODE, Shure and a few others.

Your first step is to set a budget. I do music videos with Sony HDR-FX1, HDR-FX1000, HVR-Z5 and other camcorders - with XLR mics (using an XLR adapter when needed).

It depends on what sort of music, your budget, live vs planned, and lots of other stuff... I use the cameras I use because the lenses and imaging chips are large. They provide a much bigger use-window of opportunity that consumer cams or dSLRs...