Hi John,
Linux CNC using the Linux operating system. I would suggest you buy a cheap PC to run it on rather than dual booting with a windows PC.
I won't blow smoke and say straight off there is a bit of a learning curve to Linux CNC and I'm not an expert on it either so won't be much help, but it's got a very good Forum and there are a few Linux users on this forum, Clive S is the Linux CNc guru and if you go this route I'm sure he'll point you in the right direction.
Regards the Spindle controller then it will be simple enough to re-use, forward, and the reverse is done using Contacotors that are controlled with Outputs from the controller and the BOB will provide 0-10V signal required for speed control. This is standard stuff and nothing special or difficult to implement.
The only bit that will get more difficult is if you want to re-use the Jog buttons etc on the front panel rather than using from the screen. Well, it's not actually more difficult and easy enough to do but does mean you require more I/O so you might need a different board like a Mesa card which provides more I/O. But that would be the case no matter which controller you used.!
I suggest you go to the Linux CNC forum and have a look around. Linux CNC can get deep but it's infinitely scalable with the right Mesa cards and there are some very clever people who support it.!