Yeah, it was the same with the sills on mine, it looked like a quick small fix until i started jabbing at it...ended up replacing both sills and then the cross member, the insides weren't that bad considering. Arches would have needed some work as well if i wasn't already cutting them out.
There's definitely bits that should have been fastened / covered differently as they seem to attract moisture / salt / mud with gaps in the panels where the factory underseal / sound deadening / panel protecting stuff hasn't covered...and not a lot of people bother with the underside of the car, hell its seems difficult for some of them to wash under the arches.
I'd consider patching it up if you've got all the bits handy, gives you a bit more time to decide (worst case you've spent a few hours patching for no reason, though i guess It's for me to say that as i don't have the hassle of doing it)
. While there are bound to be plenty on decent shells knocking about I think the only way you're gonna find a 100% guaranteed rust free shell is from someone who's already done the work on it (matts rally prepped one's for example).