FM modulators are horrible for sound quality.
But, whether or not it's worth it is up to you based your goals for the system. I use a similar/cheaper interface from Alpine and I'm glad I have it. It does make a difference.
Were you planning on adding an amp(s) and all?
For components, go audition as many as you can at a local audio shop. Find out what kind of sound you like. Personally, I absolutely love my Alpine Type X components for their price.
MODS: PW TMIC, Cobb catted DP, HKS cat-back, AVO filter, Bren e-tune; Konis/Epics, Advan RCII