The 04 S-Line was the LAST of the C5's 
You get an updated TQ converter, S-Line adds like 10-15hp, and you get the sports package ect.   
I had an '03 and loved it.     
Unfortunately, unless you are a loyalist, the C5 chassis is dated at this point, and will likely need suspension bushings soon. That, plus the rebuilt title will hurt it big time.