Diffuser: because of the inherent geometry of the car, no diffuser on the market will make any significant difference, or any difference at all.
The lip I would recommend is the GTS/GT4 lip. This is a part actually wind tunnel tested to be effective.
http://www.m3post.com/forums/showthread.php?t=510934
Not sure what you goal is. If it is just for cosmetics, then buy whatever that looks best
