2015 MBP is solid, but it's far too old. You would need a battery replacement and you would have to hope it doesn't have any serious issues. These are harder and harder to find today.
I'm using 2020 13" MBP for music production. This feels weird but here is a
track I made entirely on this machine. I've got the 10th gen i5 / 16gb ram variant. An i7/32ram will would be even better if you can spare the cash.
If you have NO other options and must buy second hand, then you need to get lucky with a very well-preserved 2015 MBP with MAXIMUM specs from the time, or risk it and get refurbished models from 2016-2019, as the 2020 models are still very new. Yeah, the keyboard might fail, but it might not if you keep it clean. I would still avoid butterfly keyboards if I can. Also keep in mind a 5 year-old better-designed keyboard is still a 5 year-old keyboard.
Oh. And you might not like this, but with a low budget you will be better-off with a cheaper Windows laptop. Just laying down the facts. It will give you better bang for the buck at the cost of worse design and obviously, having to use Windows.
Keep in mind Catalina models (2020) no longer support 32 bit and cannot roll back to Mojave properly. This means forget 32 bit plugins if you buy them. I can kind of live with that but some cannot and I understand them.
