WinXP version

will it work on WinXP (?!), and if so, which version ?

Have a look at: System Requirements | LibreOffice - Free Office Suite - Based on OpenOffice - Compatible with Microsoft
where it states which SP of XP is required to run LibreOfffice (and the other specs)

And take a look at 5.4 release notes, where it’s officially stated that 5.4 is the last version to support XP/Vista.

If you have an old PC you can install Linux in your PC. On an old computer Linux is very fast and LibreOffice is installed directly.
Xubuntu is a special Linux version for old computers :
Alternatively you can install Mint-Mate

Good luck !