Here is a link that has photos of the complete rocker panel as it comes from MB. MB W124 E-Class Rocker panel- Center Pillar - Ersatzteil-Ossi I actually found out from the classic center that these are still available. So if I can put the whole thing in, I'd rather do that ... my thinking is that the closer to factory design I can get, the better.
This still leaves my concern of whether to brace the car. Googling rocker repair on unibody cars in general it seems they do need bracing, although I'm not sure if that's the case when I'm not removing the inner wall of the rocker. I have the doors out which is concerning, so maybe I need to brace across the door openings?
I might be overthinking it, but I would much rather overthink it than finish it and find out I can't get the doors back on because the whole car shifted.

Simpler=better, when you did your Jeep, did you use any bracing? What was your procedure? I'm not massively worried about the welding itself as I think I can do a good job with patience. But the preparation I'm more worried.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)