You dont HAVE to remove all the old paint. In fact, it can provide a good pre-primed surface for repainting.
The best primer is grey or flat white. I prefer the latter, especially for a color like orange.
You must sand the frame well and get it very smooth, however. I always finish sand with 600 Wet-Dry grit before applying anything.
The best paint in a spray can is automotive paint. It is sometime found in large cans and often as touch up paint in smaller ones. It is expensive, but it is tough. Whatever color you use, you will always want an EXTRA can of gloss clear. There is always room for one more can's worth.