Hallo mircea
I’ve used Ubuntu-Mate daily for over two years. It’s where I want to be.
However,
on a T42 I suggest you consider installing “MX Linux” (https://mxlinux.org/).
To give you a taste of what it is about you might find this review useful:
I know of a non-profit organisation that resells hardware as old as yours - they install MX Linux on these machines.
I myself keep two virtual machines, one runs openSuSE Leap 15, the other MX Linux. But these are my reserves. I even run Ubuntu-Mate in a virtual machine at work (we all know that windoze has its limits) to help me out when I need Linux tools. 
So, Ubuntu-Mate whenever possible (for me), but when the situation requires it you are sometimes better served running something else.
Good luck with your machines, they were built to last. 