Which arduino should I use?

I need to take a serial signal from a drum hit, convert it to midi, send it to my computer as well as use that same information to trigger a light. Which processor should I get for the job? Anything else I should know? New to all this...

The MEGA has 2 serial ports and plenty memory. I’m guessing it should do

Yup - the two serial ports needed kinda weds you to the Mega, I think, unless software serial will handle it well, which would be something you'd need to study - the software serial libraries let you get extra serial ports, but they've got limitations (there are several implementations of software serial available).

That said - I don't know much about audio stuff.