There's a spot welded seam at the top of the door, if the rust is well into that you may have problems effecting a permanent repair. If it's just surface rust, then cleaning the surface rust, treat with one of the phosphoric acid treatments (Kurust, etc.), a smear of filler and repainting should do the trick.
If it is bad once you start poking, I would seriously consider a good replacement secondhand door in colour, before you start shelling out for rust treatment, paint, etc..