2018 Mini with 16gb of RAM should handle GarageBand easily.
But be aware that you CANNOT upgrade the internal SSD -- it's soldered in.
If you need more storage (for GB projects), easiest way to obtain it is to plug in an external USB3 drive.
A 512gb or 1tb USB3 SSD should do fine.
He should still BOOT AND RUN from the 128gb internal SSD, however.
It's VERY fast... faster than almost any external drive.
You could either buy a USB3 SSD "ready to go" (such as the Samsung t7, DON'T get the "touch" version), or...
Build one yourself:
- get an nvme "blade" SSD -- many available.
- get a USB3.1 gen2 enclosure for it -- many available.
They go together with a screwdriver (some just snap together).
That said, if you want another 2018 Mini with a larger SSD, look for 512 (minimum). That should be enough.