I'm not that deeply knowledgable about the non-M versions but I'll give it a shot.
There's a pretty good thread below and while it's on the 04 IIRC the Sport Package has a stiffer suspension (which some find harsh unless they ditch the run flats), and a Sport button which changes the steering to a heavier feel and changes throttle mapping. Steering and throttle mapping are probably the biggest difference, except for things like an M sport steering wheel or M sport seats--neither of which I can tell you a lot about. You can always switch to an aftermarket suspension package, or 80,000 miles, it may be due for one anyway.
As far as a repair it depends on the quality of the work. A well done repair shouldn't be a concern. But a PPI wouldn't be a bad idea on either at that mileage--they may be due for a suspension refresh, brake rotors, etc., in which case you can drive down the price or know to pass.
Prices? KBB, Edmunds--many others are the best bet as is location. Sales tend to slow in cooler climates this time of year but less so in southern states or SoCal or AZ.
2004 Z4 Sport Package - What's in it?
http://www.zpost.com/forums/showthread.php?t=327538