With 16 gb internal storage, none of the supported versions of regular Ubuntu will work, as Ubuntu installation will consume almost all of the disk space. Afterwards, you will not be able to install any programs due to lack of space.
Instead, get a fast MicroSD card (64 GB or 128 GB), and use it as your primary boot device.
As the computer is old and not very powerful, lightweight flavors like Lubuntu and Xubuntu would be much more responsive than regular Ubuntu. Also, their default installations consume significantly less disk space than regular Ubuntu. You can use the latest LTS version for longer support.
Also, if you use APT packages instead of snap packages (e.g. see this answer for Firefox), they will consume less disk space and the computer will open programs relatively faster.