G540 can provide higher current, and more voltage. This will allow you to get at least double the performance of your motors, provided you use 48V, and the motor's current rating can take advantage of it.
Even if used at the same voltage and current, the G540 will outperform the other drive, as it has more advanced features like full step morphing and mod band resonance compensation.
It's real a no brainer - G540 is the hands down winner.
The Smoothstepper is used instead of the PC's onboard parallel ports, or with a PC with no parallel ports. Since you have it, you might as well use it.
You'll probably want to get the latest Mach3 plugin from their website,
Warp9 Tech. Design - Home of the SmoothStepper - Home
The relay board can be used to turn the router on/off from Mach3.
Don't have an answer on your other questions.