Changing a processor in a laptop is not something I’d take to lightly (and I am in IT). Upgrading RAM and an SSD are not big problems. And chances are you’ll need to upgrade your used MBP with those two things, so that takes it out of the equation.
I don’t want to rehash the MAC vs Windows discussion and you can make both to do the things you need to do. However, imho, there are a few features with an MBP and OSX that make it more suitable for music related business in general and for DJ-ing in particular.
Price is an issue, agreed, but they seem to keep their value longer, even when reselling the used MBP two years down the line.