Best Options for Door Rust

I'd like to do something about my rusty doors. I won't waste time describing the degree of rust beyond saying they've rotted through to holes you can stick 3 fingers into in places but are totally fine elsewhere. So complete door replacement would be overkill IMV. At this point I would have liked to have uploaded a bunch of photos I took, but it's no possible since the site's been revamped, so you'll just have to use your imagination, I'm afraid.

Is that the worst bit?

It's representative of the worst bits, yes. Trouble is, they're not all confined to the underside of the doors! Damn nuisance not being able to upload the other pix.

new doors or here is your chance to learn to weld

Use photobucket.

New doors are not expensive, or maybe good used ones.
As they are also very easy to fit, l'd just replace doors where the outer skin has rusted through.

Let's try this:
https://www.flickr.com/photos/128859641@N02/

Quite often it's just the bottom of the doors that go and you can cut the rotten steel out back to good and weld new in. However, you have bad rust high up in the frames and so I would say it's not worth spending time and effort on them.
Depending on if you want to keep the truck, there's a range of things you could do.
Get some decent second hand replacements.
Get some decent second hand replacements. Peel the skins off, dinitrol the inside and outside of the frames. Rub the inside of the skins down and paint. Put the skins back on.
Buy cheap new doors. Take the skins off. Get the frames galvanised. Paint the inside of the skins. Put the skins back on.
I got my door frames plastic coated. Seemed like a good idea at the time and it should work well, but not as good as galvanising. Which didn't occur to me at the time! Nugget

If you have the money, galvanised Puma doors are a long term solution. If you like tinkering, buy a welder for 1/2 the cost and patch them up. You will have a new skill, as well as better doors!