I'll be a bit more blunt about it, that is shot and will only get worse no matter what you do to it. In fact the longer you leave the more rust damage will occur to the door frame creating even more work.
Now at the very least you may get away with the lower door frame return to replace, however that will be highly unlikely and will need the complete lower section of the frame replacing. Now none of this is complicated, there are repair sections available to cover that area. The two areas that you cannot buy repair sections for are the side returns. These you will have to make yourself, again not difficult.
The skin. Now this is where it can go totally tits up. Three varieties of skins out there. Pattern alloy, Pattern steel (Magnum) and good old Heritage. Now i cannot comment of alloy but would not recommend it for a daily, its more a weight saving exercise and has its own problems fitting.
Magnum skins. Now these beasties can be hit or miss. The number one biggest issue is if they are bowed at the top. This can be addressed but you really really need to be careful here as its very easy to kink the skin. However they are by far the cheaper option.
Heritage, the best you can currently get but with the biggest price tag as well. They do fit well.
Fitting the new skin involves a modicum of seam welding and some spot / plug welds. What you will need is a reasonable quality panel beating hammer and dolly. You can buy a kit for somewhere around the £40 to £50 range.
Have a look at the Project Paddy link in my signature or look for Wills Mini Special or in my signature The Red One. All of them show what you will need to know about door skinning.
There are several tools out there you can buy for skinning doors BUT the hand manual one leaves marks on the folded return (found this out when i bought and used one) the over is a pneumatic type https://www.google.c...HYDD8dsPJT0hQM:
Been recommended one more for speed than anything else but i am not in a hurry when doing these jobs.
Now there is one more panel you can buy without removing all of the skin which is just a lower section of door skin. The one biggest issue with this puppy is you will get distortion of the upper skin, plus realistically you will need a joggler. Having done one of these on the wifes old Mayfair i would not do one again plus also with what i know will be hiding on the rest of the skin.
So if you need any tips holler out.