I want to run music maker premium 2021 well on a new/refurbished laptop but only have a budget of about 300. What laptop to get to run it well? I won't be using it for anything else.
It depends on what you want to do and what you already own. Record your own vocals/instruments or MIDI? First, you'll need Windows 10. Do not buy a laptop with a Intel Celeron or Pentium. These processors are really slow. I think it should be at least a Intel Core I5. You'll need a good amount of RAM. I would suggest 8 GB RAM at least. Windows 10 needs a lot RAM and your songs will too.
If you want to record your own vocals/instruments you'll probably need an audio interface. A cheap one would be the Behringer U-PHORIA UM2. For MIDI recording you would need also a cheap MIDI USB controller or keyboard.
If you go to this page you will see the minimum requirements for a computer to run MM. But remember, this is the minimum. Ideally, you need to be looking at a machine that exceeds those minima by some margin at least.
In addition, as @SP. says, so much depends upon your actual usage of MM.