Safety / Stability of Ubuntu?

[grahammechanical]

However, I'm worried, as i had previously attempted to install Linux Mint on an old PC

How old? It could be that Ubuntu might not be the best choice for that old machine. There are what we call flavours of Ubuntu which use a different desktop environment and user interface than Ubuntu which uses Gnome desktop environment.

Flavours such as Xubuntu and Lubuntu might work better on that machine than Ubuntu because need less resources.

https://xubuntu.org/

https://lubuntu.me/

Because they are official flavour of Ubuntu they are as stable and secure as Ubuntu.

Regards

[Norm24]

Something that helped me a lot at first was the book The Linux Command Line by William Shotts, you can get it as a PDF at https://linuxcommand.org/tlcl.php

Excellent reading.