it really doesnt matter if you build the motor first, or put together the chassis and small parts first, because either one wont work without the other lol.
+1 on buying an esr builder kit and sending it to a reputable builder such as arlan to set it up the way you want.
it may be cheaper in the long run to find an ebay parts lot that has most of what you need in it, rather than buying every little small piece seperately.
also instead of trying to mess around with used electrical, just buy yourself a cr ign setup from the get go, and never look back.