I'm only stepping in here because no one else has answered. I'm new to this as well. The higher the voltage, in a general way, will give you more speed and more heat to worry about. So before you even pay to upgrade the power supply, you should determine the speed you are looking to get and with the torque you will need, you can check your motors torque curve to see if you even need to up it to a 48v PS. No need to produce anymore heat than you need.
And as for the second power supply for the relays, you should check the schematic, the second PS is probably a 12v or 5v for the relays.
With any luck, some of the more experienced guys here will chip in with a better explanation.