I don't know about the 650w motor specs, but I bet it's pretty close to the 750w. My Son's bike went like ~24mph with the stock MX dirt Rocket gears setup(80tooth rear/11tooth front = 7.27:1 ratio)
*Because you are now going to buy the MX650(650w motor), this would be a good candidate to first just try the 48v upgrade(without trying the 750w motor at first). It's only advertised at 100w less than the 750w motor... If you go onto the unite motor web site, they don't even list this motor? I am personally wondering if the MX650 may really just have the 750w motor in it.
Filitup, this might be worth holding off on the 750w motor, to see if you get the same performance at aproldan and I did, before you spend the extra ~$65 on a new motor, that you may already be getting with the mx650...
If you just go without the motor for now:
~$20 Extra 12v/12ah battery.
~$40 100a/48v controller
~$38 48v battery charger
~$45 for incedentals, and shipping..
~$140 - ~$150 total for your initial upgrade..