Should I buy this truck?
 

I would walk, if there is rust that high in the cab then god only know how bad it is lower in the cab.

Yeah, I'd walk. It's a goner. Rattle can over rust. It's got rust in places a flea couldn't get to.

AFAIK they do NOT make patch panels for that area of upper B pillar, yes they do sale repop lower cab corners patch panels.

if its almost free go for it.

Even if its free, if your plans for it are anything but a parts truck, it will be nothing but frustration and a money pit. The rust you can see is the tip of the ice berg. I understand anything can be fixed but with decent trucks around for reasonable money it doesn't make sense to start with something like this. Really even if it was given to you it wouldn't be a wise venture. Be patient and keep looking

That area can be fixed if your any good at metal working an a welder but the only issues that I would look closer at is around the windshield openings , if windshield area is in good shape , then the other area rusted if that's all you found would not bother me ,,look closer inside and out for rust problems but only the windshield areas would be a major problem and I would walk away depending severity ,,may be hard to see if the windshield is still in it but check it close an lift up the rubber some , look for pitting or any rust around rear an front glass channel areas , front is the worst to fix

Yes, do take a walk unless you are keen to challenge your fabrication and restoration skills. That kind of contest (rustiest start) might make an interesting YouTube channel or reality TV show.
That is, until someone starts reproducing the truck bodies of this era. Already, we can buy new steel bodies for a '40 Ford, '67 Mustang fastback and all manner of Chevys. As I understand it the steel is stamped in China and the pieces are assembled in the USA.