She is real sexy now you have her all stripped down.
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I replaced both Trailing Arms on the 85 300D when I was doing the Sub Frame and trailing arm Bushings. I found them on another 85 a 300CD with just over 200K. got them both for $42 ea.
I just left the bearings in that they came with, figuring they were new compared to my old 350k miles ones. they still are running good. swapping in lower mileage trailing arms was easier than changing the bearings.
Now you have them down to bare metal, degrease them real good with Marine Clean, treat the metal with Metal ready, and throw on a couple coats of POR 15.
I POR`d mine, but being in Calif and no rust issues, I degreased, sanded the arms and sub frame down and painted with POR.
It looks rusty in the splines,but I have seen that before, and maybe in your area with the salt might be more so. I would slap on the anti seize before installing the axles.
Welding on the arms would not be a good idea. with some pin holes showing, there is probably more rust on the inside.
there was a couple threads with pictures showing cracked arms from corrosion. not a pretty picture.
Charlie