I glossed over things like wheel offset and whether it centers on the bolt circle or on the hole in the center.
Will you let the projected savings dictate a budget, or cost out the swap and see how long it takes to pay out? Costs depend on a whole lotta things, new/used, etc. I suggested junkyard parts.
If you look at the car's energy budget, rolling resistance and friction go up with speed on a flat curve, but aerodynamic drag is on an exponential curve. So reducing rolling resistance offers returns at low speeds that aero doesn't.
It all starts at the tires contact patch. A tall, narrow tire on a light-weight rim gets you off on the right foot. Every other mod benefits.