Try posting this in the Bodywork forum under Do It Yourself Links and Resources.

To address your concerns anyway, the repair method and cost will be dictated by the extent and location of the rust-out. The side sills, or inner rocker panels, are an integral component of the unibody structure.

A larger section like an entire floorpan, will of course require complete removal of the interior in the vicinity and everything underneath that interferes with cutting out the old and fitting in the new. And, all the undercoat and soundproofing will need to be removed around the joint as well. The sealant and undercoat is very flammable.

Labour to do the actual cutting and fitting and mig-welding is specialized work, but you can do the parts removal, stripping and cutting out the old yourself.

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1986 560SL
2002 Toyota Camry
1993 Lexus