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.
If I understand this discussion so far. member Andy is the only one to maybe poo poo aero IMO.
To restate my point, on my original posted priority list for areas of concern for the OP's project, aero is the LAST issue of 9 he needs to deal with.
I never said nor implied to anyone it has no effect or nor importance at any speed.
As I believe as a matter of fact, and supported by my posted builds here over time,, IMO, I'm indeed a huge believer and follower of Aero solutions.
Strangely I suspect and noted just for conversation sake, at the F1 level, my list is almost completely reversed, but then of course, this is not F1, stating the obvious.
