Well, aero matters from about 60 on up; that was proven by Car and Driver decades ago in a fairly thorough experiment.
That said, there's a lot more to be gained, in larger amounts, with the basics, as already pointed out.
OP needs to decide on his budget, and I'd advise developing skills before spending $$$ building the car.

The aero gains on that body primarily involve the deep & open grilles and a profile that benefits from a rear spoiler.
I'd flush or block the upper grille openings, use the valance for intake, add an air dam, and vent the hood.
A rear spoiler that extends up from the upper rear face of the trunk lid works pretty well; that face has a decent angle.