Controlling spindle speed via software

You need a spindle motor driver. It might not be a VFD, but you need something that can handle the 300W of motor power.

The BoB has a 0 to 10V output, but that can only drive a few mA of current, enough to control an external drive.

Well, typically the controller will output PWM on one of the DB25 pins of the parallel port, and then the BoB converts that into an isolated (hopefully) 0 to 10V signal.

Part of the reason to do this is that the potentiometer pins of some drivers (KBIC DC motor drivers for example) sit at 200V above ground. (ie the control is 0 to 10V but is actually +190 to _200V relative to ground or neutral) If you wire that directly to a parallel port pin you will need a new computer.

The BoBs contain opto-isolators that use pulses of light to transmit logic signals between the delicate PC parallel port and the high-voltage, high-power electronics.

What are you using as a spindle driver?

And i would still recommend using the BoB to isolate the driver from the PC even if you choose to ignore the 0-10V output converter on the board.

You could use the first one if you ignore the PWM to voltage stage on your BoB. (You can still use the relay)
You should be able to output PWM on one of the other outputs, if the PWM pin is not broken out independently.