Hello!
To your questions:
1. The best way to remove old painture and rust from the parts is brushing with fine sand and (medium) pressure. But - after that you have to paint it as soon as possible with any kind of primer against new rust. The humidity in the air ist enough to have a fin golden-brown rush after a short wile.
2. Your second question is hereby obsolete ;-).
After brushing with sand you have a fine pitting on the surface - so you have to fill it with special filler to produce a good finish before painturing with a color painture. OK - the sandpaper has to be your best friend after fillering and before painting.
My way is to use first grain size 400 and for the last grinding grain size 600. That should be enough.
3. Colors - we have had some discussions about that. RAL tones are almost not applicable. One exception: For the "maroon" color you can use black-red RAL 3007. I don't know the ISO standard for this color or if it at all exists
Here's a link about our discussions:
ColorsIf you have further questions about the discussion don't hesitate to ask ;-)
Greetz
Daniel