Personally I like using that filler primer when using rattle cans, ya still gotta spray it on in thin coats and sand in between coats, but it's great for smoothing out imperfections in thte metal like minor pitting or smoothing over welds depending on what welding method was used during manufacturing. 2 of the 14 ounce cans will be plenty enough...
Remember, the paint is only as good as the primer that's under it so don't take shortcuts during priming, and the primer coat is only as good as the surface under it so proper surface prep is what's going to make sure the primer sticks and never peels or separates.
For the color coat, 2 cans will do and you also want to apply several thin coats instead of fewer thick coats, sanding in between coats. If you're using metal flake, prismatic, or pearl colors, do NOT sand because it'll ruin the effect, just spray on thin but wet coats, let dry, then resume sanding in between coats when applying the clear.
The clear coat is what's gonna give it it's shine and protect the color coat, I prefer automotive grade 2 part clear, and if you don't have a paint gun you can still get the same results by using 2k clear that comes in a can, it has a mix charge under the can and gives you about a 24 hour window to spray it, this stuff costs about $30 per can but will give far better results than any single part rattle can clear. This stuff is actual automotive grade clear in a can and it's used by paint and body shops for small repairs etc where using a gun isn't practical, it's a little expensive, but it's the next best thing to having it professionally done at a paint shop... http://www.spraymax.com/index.php?id=361&L=1
They sell it at most auto paint supply shops but you can get it off Amazon as well... http://www.amazon.com/Spray-High-Glo...+2k+clear+coat
This stuff is thick so one can will most likely do the trick. I use about 4 ounces of clear, 4 ounces of reducer, and 2 ounces of catalyst to spray a motorcycle tank so a 12 ounce can of this stuff should be enough to cover the frame and fork.
Another option to having an automotive clear coat would be to talk to some of the local paint shops in your area to see if they will spray on the clear for you, most of them will charge $20 to $50 and just put the frame in with the next car they shoot since it's just a small amount of clear and spraying. You'll end up doing the sanding and buffing, but that's where you save money, and if the painter is really good it may not even need to be sanded down or buffed out.