I wouldn't advise recycling the prebuilt's case. In fact I'd suggest that you look into a good airflow, cheap matx case before thinking of an matx B450 chipset motherboard.
I already changed the power supply to 650 or 700 not sure which.
If you're unsure, how are we supposed to help you? At the end of the day, you'll be working with the system you're looking at, so it's best you're brought up to speed with what you have to work with, of what it can and cannot do.