I have an old PC case with - Intel Celeron G465 1.9 GHz single Core processor;
- 2 GB RAM;
- Intel HD Graphics 256 MB on board;
- 250 GB hard drive. installed, What would be needed to upgrade it to a sensible KDE machine Kind Regards Geoff

The problem is the processor: it is only 32-bit, single core and "Celeron", meaning with a reduced chip-set capability. Celeron processors have always been a bad idea, and I can't imagine anything making this computer better but to get another processor. It is really outdated and will slowdown everything. Regardless of what RAM or Hard drive power you ad, this will be the bottleneck. The other slowing factor is the graphics card, but with that processor it is not worth investing into anything faster or newer.

Of course it is possible to install a recent Plasma on it, but it will be really, really slow. Up to you if you want to go through this. The base distro needs to be 32-bit, and the only current distribution that still offers 32-bit kernels is Linux Mint.