Since it is dependent on the nature of the problem, whether you choose new or used parts, the person performing the repair, and so on, I am unable to provide any advice. In any case, you won't know for sure until you open the door and take a good look.
However, if you are unsure of the vehicle's overall condition, I recommend taking it in for a MOT regardless to find out what it fails on, especially if the inspection is imminent. It would be a waste of money to fix the door if it turns out to need far more expensive repairs and you end up selling the automobile. The obvious caveat is that I don't know anything about the car or your situation, other than the fact that it is a Mark 1 Focus.