If you just want some fresh paint, scuffing the old paint off is fine. Do take all of the cheap factory clearcoat off, though, if it hasn't started checking and peeling yet, it will as soon as the new paint starts to dry. Getting through the clearcoat layer just to the basecoat will be obvious after you've sanded a panel or two.
If you want to do a job that's going to last a long time, and you're trying to save on labor, remember that surface prep is 90% of the game. Sand it down to the electroplate primer from the factory (it's usually black or very dark gray, and is always the bottom layer) , but no more than that. Then:
(1) Do any body work (parking lot rash, etc.) needed. Take your time with this.
(2) On metal, conversion coat any places that the sander might have scratched through to the metal.
(3) Epoxy prime. Do not skimp here. Etch primers are cheaper/easier, but have much less chip and rust resistance.
(4) Fill prime using the two-color method, and block sand.
(5) Base color. If clearcoat, just enough to cover, not much more.
(6) Clearcoat (if chosen).
If you're just shooting for cost, stick with an acrylic enamel. If (when
you make some mistakes, they can be corrected without too much swearing. Lacquers are cheap, forgiving, look great, and are easy to repair, but durability is useless for any daily driver. Nice for show cars only.
If you really want better durability, a urethane topcoat can't be beat, but it's (a) expensive, (b) unforgiving; if you make a mistake, you may have to re-do the whole panel, and (c) it requires a full pressure breathing rig, because the isocynate vapors can cause some really nasty reactions in your lungs. Best to rent some time in a spraybooth if you want this.
Let's see, what else?
* Try not to mix paint brands/processes, stick with one process if you can. I had a horrible time with Imron over a Sherwin primer on something I did recently, and now it needs redone in 2 spots. It was a complex fiberglass shape in 3 colors, took me two weeks prep, and then it ran because of adhesion problems (I panicked and overdid it). I'm still ticked at myself...
* If you take the fiberglass tailgate down to the glass for whatever reason, definitlely take your time finding pinholes in the resin with Featherfill. They sometimes don't show up until the paint cures several days later. Also, use a primer with a UV blocker on there, because UV will destroy the resin over time.
* Get some books, like bustednutz mentioned. Start with your local library.
* If this is your first time with a paint gun, practice on a junk panel or steel sheet before you shoot your real rig. The guys that shoot paint everyday have a touch and an eye that only comes with lots of practice.
* Paint brands: easiest for you to find will be PPG, DuPont, Sherwin-Williams, and BASF. Go to their websites and read the PDF instruction sheets for each process. Cost is competitive for all (always too much), what you should get will depend more on what your local paint shop has available. A couple of decent, affordable acrylics are DuPont's "Centari", or Sherwin's "Western". The more expensive paints will last better.
* Did I say practice?
It's hard, nasty, expensive work, and will take way longer to do than you expect, but the results are always fun to admire, and the feeling is great.
Sorry this got a little long. I love to paint, unless I'm actually in the process of painting...