Having restored many Mk1s over the last 20 years I'm not afraid of encountering rust in any area of the car, sills, floors, door posts, roof sections etc can all be repaired easily.
However that area in question has seen many a Mk1 off to the breakers yard, the reason being you need to cut the lower suspension mounting point out to repair it properly, as the rot is right in behind it.
Now a quick plate up job will probably see you a year or two more out of it, but eventually the entire mounting point will rot out and is actually very unsafe well before that happens, imagine that lower mounting ripping off the chassis at speed.
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it doesn't bear thinking about.
The problem trying to repair that properly means the mount needs removing, so you lose the datum point of where it used to be after repairing the inner chassis area, you would need to make a jig to catch that bolt hole before cutting it out, repair all of the rot and use the jig to weld a new mounting point in.
The cost of doing something like that outweighs the value of many Mk1s and they inevitably get scrapped.
I normally call time on any Mk1 I see that has rot in this specific area as there is no easy way to rescue it long term.
