Well, this is what I've found out about Roush since I bought mine 3 months ago.
First of all, I know they offered the 6cyl every year from 99-04 but it was mainly an appearence option (ground effects and exhaust). There were optional upgrades that included suspension, brakes, and I believe a cold air intake was one too.
Now here is where I get confused. My Roush is numbered (#1047) in the door and on the dash yet there are no records of how many were made!!! I even called Roush and they told me they didn't start keeping track untill 2002??? I'm still not sure how you can number a car then not know how many you made!! WTF!!!

Further more, they are discontinuing most replacement parts for ALL 99-04's!! I heard that they are doing this because they want originals to become rare and more valuable. That sucks for the people that drive them though! If you can get Roush OEM parts they cost a fortune!!! I just replaced my brake rotors and the Roush rotors were $400 each!!! I bought the Powerslot rotors for $100 each! The Roush rotors are probably a little better but not 4 times better!! I also found that the Roush guys usually think they are better than everyone else and I hate that.
All that being said, I love my ROUSH!!! I have the stage 2 (8cyl) with suspension, brakes, and CAI. The stage 1 was a body kit and exhaust. The stage 3 was all of the above plus a supercharger (unavailable on the 6cyl). I personally think it is the best looking
stang in those years. I was at Limerock Raceway over the weekend and there must have been a 1,000 stangs there. There was only 2 Roush, mine and a brand new one on display. When I went to leave there was a bunch of people standing around my car pointing at the exhaust. Also, my Roush stops and handles better than any other
stang that I've owned (I've had 12).
If you want a
stang that sits apart from others than go for it!
Let me know if that helps. Good luck!