Buying the rear wing lets you adjust rear downforce. So far I've not see any front/side/extension aero parts that allows you to adjust the front downforce. Only in my F430 Scuderia where the aero kit would give the front downforce a fixed value of 15.
Also to answer the above question: In GT4, buying a rear wing allowed the adjustment of front downforce. It was probably assumed that it would be the same in GT5.