Forget paint, grinders, rust treatments, etc...
Go for the quickest, easiest route. Find a door the same colour in a breakers yard. Many yards have a list of all the cars and colours on a computer, so you can start by phoning them, and they should tell you "Yes, we have two blue ones" or "We have a silver and a red one". They may not be able to tell you the EXACT shade of paint, and your car may have been available in pale blue, medium and dark blue, for instance.
Unless they supply you with the key, you will need to get your own door lock fitted, and of course, your own cars' interior door trim. Make sure the glass is tinted the same shade! If you have green tints, and the door has brown or grey, you can have your own glass moved over into the new one.
It can be very cost-effective to simply change the door, and could be a done-deal inside of an hour, rather than you putting the car in for paint.