For the question about the fenders:
Yes they had to be changed due to the difference in the shape of the headlights. The 96-98 fenders would not match up correctly since the shape of the 99/00 headlights have a more lined shape and not a circular angle like the 96-98s did.
For the question about prepping a car:
Yes it is something you can do yourself. I am taking a class at a local community college through my high school (they are paying for it since it wasn't offered here). Read below and I will tell you a basic over view on how to prep it.
Taping:
It is basically what it says. Tape up all weather stripping or anything you do not want to sand down. Use 2" tape for the larger areas and 1/2" tape for smaller things like wiper fluid nozzles. Make sure you get 3M tape from a sherwin williams store or some other store like that. While you are there pick up 400 grit sandpaper, a sanding block, grey scotch bright, and red scotch bright. Also, if you want to spend the extra money to do it right buy a tube of "Scuff and Clean."
Sanding:
It is something that you have to practice before you get it right. The corners of everything that has an angle (the downward angles on the bumper etc) will have significantly less paint on them than a flat surface. With that said, the way I am prepping mine is I get a bucket of water, wet the area down that I am sanding (AFTER I HAVE ALREADY TAPED THE CAR OFF).
Once the surface has some water on it I take grey scotch bright and apply a fair amount of scuff and clean to the pad. Go in circular motions all over the place that you are sanding. If you are going to just spray clear coat on something use the red scotch bright and scuff and clean.
400 grit sand paper and a sanding block is used when there is primer or you need to sand off layers very quickly... be careful if you do it too much you will go to bare metal and will have to reprime everything.
Priming:
If you prime something and it isn't using an actual paint spray gun make sure you use a urethane based primer - 3 layers usually. After that take the 400 grit sand paper (wet sand using the bucket etc. constantly dipping the sand paper in the water to clean off the primer/paint etc.) and sand down carefully - be sure to not go through or you will have to reprime and sand over and over and over until you get it right.
If you have anything else you want to ask just let me know. Before you can even think to prep the car you must make sure all dents are taken out using either bondo, plastic filler, or simple banging to get it out.