Body work/welding

I was just wondering about opinions on the best method to patch and weld along seams. A few pictures

Is it best to just cut the piece out and rebuild the metal or will I be fine to just add bits and pieces?

If there is rust then cutting and replacing is the only way to go.

While you’re welding already, cut out everything that can be repaired.
I only welded once but you can replicate the spot welds by drilling out and plug welding, and take your time with the rest. With what you have first remove everything questionable and then you’ll see what needs replacing. Small bits are okay here I think but it depends on how much you find. You can look inside the sill by taking the end cap off from the wheel well.