Not sure if rust on these suspension parts warrants replacement (structural weakness?) or if I can just wire brush them down to bare metal and primer + paint?

 

If the ball joint is worn out, replace the whole arm. Otherwise, just clean it up, base with Eastwood rust converter or POR15, and topcoat.

 

It looks like a bunch of surface rust to me. Could use something like Purple Power( same as Eastwood but cheaper) which you can purchase at an Advance Auto. Then I usually wire brush because a lot of times after using the derusters the surface isnt very good.

 

What's the best way to tell if it's worn out?