SSD is the most expensive cost per GB there is. I have read up on them a little, and I like the fact that they have less moving parts (none?), so they will run cooler and they will function more reliably in an application where there may be more vibration, like a tractor cab....

I have an old computer like you describe that I still use everyday. I installed a second regular old hard drive in it as a slave drive. That is where I backup many critical data files regularly.