Right down here, I'm going to tell what each part will modify on your car!

Paintjobs:
1 - Stickerbomb
2 - Carbon
3 - Triangle pattern (blue and green)

Exhausts:
Alien - Titanium exhausts
X-Flow - Neon tip exhausts (omg)

Spoilers:
Alien - GT Wing
X-Flow - Futo GT Drift Wing

Front bumpers:
Alien - Paintable front bumper with paintjobs
X-Flow - Exposed intercooler not paintable with paintjobs

Rear bumpers:
Alien - No plate, paintable with paintjob
X-Flow - Plate, paintable with paintjob

Roof: Protip: Do not spend money on these, they won't crash your game, but it's a waste of money)
Alien - Nothing
X-Flow - Nothing

Side skirts:
Alien - Paintable with paintjobs
X-Flow - Nothing

Have fun pimping it up!