I've said it once and I'll say it again: rust is just another issue to deal with, and there is no more reason to avoid it anymore than there is to avoid mechanical issues. That's not to say seek it out by any means, but almost all rust issues can be dealt with. The skills, and tools and means to deal with them are different, but doable. Most of us feel comfortable, and some even confident, with wrenches in our hands; less so with plasma cutters and TIG welders. But repairable rust it isn't any reason to avoid an otherwise mechanically sound car.
I had a lot of metal work done on my car due to the PO's (which happened to be my Uncle) lack of care, but I also had a lot of mechanical work done too. I can't say one was any worse or expensive than another. They both took time and money, and specific skill sets to accomplish.
It costs nearly $10,000 to remove, rebuild, and replace a motor today. It's over $1,000 for a FI pump. Changing all the brake lines, and rebuilding all the calipers and getting it all back together isn't chump change either. Anyone have a transmission rebuild lately? Last I checked that was 8 hours labor just for removal and reinstall (by a pro) and well over a thousand $$ for a rebuild. Anyime you tackle a reasonably major mechanical issue on any average car of this vintage you are bound to find quite a few other things that need to be done before you can safely put it all back together again. So, if you see a relatively rust free car that is smoking out the exhaust, and has a fuel injection issue too, don't assume it is automatically better or less costly to deal with than a nice running car that needs a new trunk floor and wheel well arches.
There is always this elusive quest for a "rust free" original car where the mechanical items are what needs work and the body perfect. Keep looking. I hardly think these exist, but maybe somewhere they do--someone who never changed the oil, never maintained the brakes, etc. but religiously washed the car and garaged it all the time, and one that never saw winters.
I've seen a lot of body work with new sheet metal done on W113's, not only on my car but on others. It's nearly all doable. Thanks to great folks like Tim Kidder at K&K, most metal is available.
Bottom line is to really know and understand what you are getting into--and how the problems are dealt with. Rust is one issue, so are mechanical items.
Now if the car is not sound mechanically as well as full of rust, that's a whole 'nother issue...