I'm getting to a good stage with Mach3 but wonder if I should at some point start looking at Linuxcnc for the future. Not something I want to do but thinking down the track.
If you want to keep using Mach3 indefinitely, and you have a PC that works well, just buy 1 or 2 spare motherboards for it on Ebay. You should be able to find at least one for a pretty cheap price.
You can also get new copies of XP on Ebay, if you don't have a disc.
You can also buy an external motion controller like a UC100 for about $125, and run Mach3 on any modern PC with any modern OS, for as long as you want. This probably makes the most sense.
If you decide to move on to a better control than Mach3, there are other options besides LinuxCNC and Mach4.
Look into Planet CNC, Eding CNC, and UCCNC. All are arguably better than Mach3 at this point in time.