As for Linux operating systems go by, I run Kubuntu on a server that’s just a desktop from around 2010 (used from a office) and it runs fine.
Ubuntu Mate looks good, Kubuntu is however, a bit better in terms of the interface.
You can try out the OS from the USB for as long as you like, and you can just install it onto the HDD rightaway!.
Mate and Kubuntu are all run by the same type of Linux, Ubuntu so it’s really whatever tickles your fancy.