Find a tame "Smart repair" guy. There is a great one near me who does top work and is a lot cheaper than a bodyshop due to less overheads. The biggest thing they will usually do is a whole panel (wing / door etc). Just make sure that you see some of their work beforehand to make sure you have got a good un. Have you got access to a garage? You will save on labour by bare-metalling the door yourself. Use something like Por Metal ready to remove the rust and as a base for primer. Don't forget to do the insides of the door too if it is rusty inside (probabally is).