The big differentiation between whether an EV is purpose built is in the exterior design and, more specifically, whether you're primarily concerned about drag or weight. For a traditional ICE design you're concerned about mass, aerodynamics, and rolling resistance. For an EV you're looking at aerodynamics, electrical loads and then mass and rolling resistance. The Volt, the Leaf, and the Model S are all purpose built EVs. Only one uses large format cells placed under the seat but all are built to minimize drag.