They are pretty simple to fit..they bolt to the chassis and then it's a case of connecting up all the wires between the motors, control box and battery...If you're handy with tools, i wouldn't contemplate paying someone to do it for you.
And from my experience, Preloved is a better place to be looking for a mover - on ebay, even the old tatty ones seem to command a much higher price than can be found elsewhere.....Especially when you consider that it's unlikely you'll be able to test it before you buy