Tips on where to start with rust repair to my doors.....

Back into the shed and i have finished prepping the rest of the panels, now I'm onto the doors... At what point do i start searching Gumtree for better ones?
I have never cut and welded before but I'm keen to have a crack. A friend has a welder and isn't bad on it but he has never done this type of work.. Can some one tell me where to start or a good thread i can read on door repairs.
The door skin:
The inner :(
The bottom seam

All 4 of my doors share the same rust...

Thanks Jenko