The operating system and any updated applications going forward
may consume a greater part of 128GB. Usually try to have enough
free storage drive capacity so as to not fully load the computer up.
My preference is to have at least 40% free space, in the hard drive.
Newer Mac portables usually aren't end user upgradeable, either.
So you may consider up-front buying the greater capacities in the
build model version with adequate RAM & also larger GB storage.
Newer SSD technologies cost a bit more, and proprietary ones inside
a carefully engineered Apple product may costly, if possible, to further
upgrade later. A few third-party solid-state storage items are out there.
But may not be appropriate for your expected usage. And add costs.
You could get by if you were to maintain an offline storage backup drive
and a second one for your overflow. Time Machine and clones, etc.
Good luck & happy trails! 🙂