I generally dont start with one part in particular.

Just a pick a part and go....swear a bit.....come up with a solution.....swear somemore....solution number 2.....Hoorah!

But seriously if your balsa wood mock up fitted everything as it should and you are happy with the design then I would start manufacturing the chassis. If you are going for a frame with armour bolted on then start with the frame, if you are going for a monoque (armour and chassis combined) then start with the base and go from there.