If you want to go inexpensive and are not going to worry about a restoration quality job, here is what I would do.
First, you will have to get as much of the rust removed, a light/careful sandblasting, wire brushing, sanding, or chemical neutralizing. I like the POR-15 brand of products and used them on my car. I did not use any of the rust neutralizers as I did sandblast my stuff.
I would use the POR-15 products for the surfaces inside the door. You don't want to use this on the outside skin where you have the rot showing in the door corner. You want to be able to sand or wire brush this area to bare metal. You want a bit of a rough surface, not smooth, so use a coarse sandpaper, etc.. Then I would use a fiberglass cloth/fiberglass resin to create a patch that will cover that rusted hole. You could use a little wire mesh shaped/formed placed over the hole just to give your fiberglass cloth some backing so it does not drop in like a large crater. The cloth is kinda thin, so I would let the first layer cure-up and dry, then rough sand to smooth out the contours and then put another layer of cloth/resin over that to build up the thickness. I might also consider, if I could get my hand in there, cutting a small piece of cloth to fit on the backside of the patch, dip it in/soak it with the resin, and then apply it to the backside of your door patch. It will help to hold the patch from the inside.
Once done, you can sand the cloth with a fine sand paper just to smooth it. You don't want to sand too much and thin out the fiberglass cloth layers. If needed at that time, you can then apply a thin coating of Evercoat brand "Tiger Hair" fiberglass filler over everything. (Plastic bondo will probably be too hard/brittle and closing/slamming the door may cause it to crack or break away). Then let dry and do one last sanding with fine paper to blend it all in and smooth thing up. The you are ready for primer, a little more fine paper sanding (wet sanding is preferred here), and paint.
Depending on how bad the lower part of the doors are, you might want to put a layer of cloth on these as well and do the same procedure. If not too bad, once you get this area down to clean metal, follow up with the POR-15 rust inhibitor and top coat - then it will be ready for paint. Make sure you do not block the small drain holes in the lower door otherwise water will remain in the door and not be able to drain and the door will rot out on you.
Now these are just my thoughts from looking at the pics and your wanting to do a reasonable repair. I have a door on my '68 Lemans that looks like yours in the corner and this is what I am leaning towards myself. I can however weld, and may mig in a sheet metal patch and then follow up with the Evercoat to shape my door corner contour - I'm not going restoration perfect as this is a driver and I won't be selling it. But I will definitely go with the POR-15 route on the inside of the doors to kill all the rust and protect them from any future rust outs.
Hopefully you will get some other opinions and ideas on what can be done.