New Mill Build...Can I use LinuxCNC?

Yup that VFD will work with Mesa 7i92 to it's full speed, it will also work with a parallel port but not up to full speed, most probably as most of the parallel ports in the last 15 years do 3.3V, not 5V.
And you can use some "arduino relays" to do the ON/OFF and direction change on the spindle, they are about 3$ each or in groups of 4 or 8.
Since you have 3 drives that will need 6 pins plus 1 for enable (might not be enough to drive 3 drives), so that leaves 5 outputs, 2 or 3 for the spindle, so enough outputs, and you can do all limit and home switches to 1 input leaving 4 more for probe or something else.
You can remove the BOB or parallel port at any time in the future and just plug the 7i92, the difference is with that BOB you will be limited to around 10000 pulses per second, directly with parallel port from 20000 to 50000 depending on how god the latency on the PC is, and with the Mesa it can do up to 10 million pulses per second. That is very nice if you use a lot of microstepping on the drives so the motors work much smoother.
Again, the best solution would be 7i76E, and yes you can use the 12V for the field power, the same one powering the vents.

You can use the same 12V for powering Mesa.
And yes you should use the enable inputs with Mesa, but you can add those after you get the machine working. It is not required, but it is nice to have.
RS485 will simplify the wiring, but the hal setup is a bit more complicated.

Mayberry wrote:

...can anyone elaborate on the difference between the 7i76e and the 7i96.

Since i never used a 7i96 i am not best suited to reply, but as far as i know 7i96 has less I/O and is better suited for plasma while 7i76E has everything needed for a mill and then some.