I've used both Dupli-Color and VHT Vinyl Dye on several different items, everything from motorcycle seats to automotive carpets (which are usually made from a synthetic material) to CRT monitors and plastic computer cases. Vinyl dye is good stuff; just make sure your seat / saddle is clean and dry, and apply several light coats of dye. The dye tends to be "thin", so it will run MUCH easier than regular paint; spray light coats, let them dry (20 -30 minutes) and spray again. Don't worry if the coverage is spotty at first, you can correct that as you add more coats.
Vinyl dye is much more flexible than paint, so it won't tend to crack or chip the way paint does,
As for painting the tank and side panels, start with a side panel and learn as you go along. Better yet, get some old sidepanels from a boneyard, and use those as your guinea pigs, Once you're satisfied with your work, paint your "good" side panels. The more experience you gain, the better prepared you will be when the time comes to paint the gas tank.
Good luck with this; getting started is the hardest part. Start small and simple, and work your way up. Do NOT try to do everything at once, or you might get so discouraged that you end up quitting.
Q. How do you eat an elephant?
A. One bite at a time.
Keep that in mind, and let it guide you...