OP:
As others have mentioned, you don't need an SSD for an external backup drive. A spinning HDD will do fine.
Some other thoughts:
You -do- need a 2.5" form factor drive, that is easy to carry along with you if you're traveling.
You -do- need to use a cloning app (such as CarbonCopyCloner or SuperDuper) to maintain a fully-bootable backup that is "ready for immediate use" if you're traveling and experience an "I can't boot!" moment with the internal drive.
If you were using only a Time Machine backup, and had booting problems with the MacBook, you can get going again, but doing so will be more problematical and take a considerable amount of time.
With a cloned bootable backup, just connect it, and you're back in business...