I could say do the painted bumperettes first.
Lowering really is not practical but if you must, I would make it second. Then side skirts, then the diffuser.
But I would invest in some bumperplugs and possible upgrade the bumper, or get a clear bra before any of the other ones, except the painted bumperettes.
And if you check out automotovetouchup.com they sell the correct paint relatively cheap so if you've got an air compressor and a paint gun you can do the painting on the bumperettes, side skirts yourself fot like $40-80.
Just my opinion!
