How bad can the door rust be to the cars health?

This particular rust is right under the driver's door on a '90 245 wagon...

I am thinking of buying this car, but what should I do to fix the rust? A new coat of paint? Add a steel plate behind it...?

Thanks,
Conrad

PS I have added the max limit of the attached file...I would have rather wanted to just put the whole image but it didnt let me

 

I'd change the whole door. If you are near one of those Pick Your Part places, older Volvos are plentyful and the price is like $30. Or any jukyard for older cars.
But I wonder how much more rust there is in other places. Remember rust is like an iceberg-90% is hidden from view.

 

+1 on that.... There's a reason why some call rust "Cancer"...... Old VWs' are notorious for that, as are some old Volvos. I would look over the car thoroughly and see where else it has spread, that way you know what you are getting into! If it's just the door,you could weld a new piece, grint the rust and use body filler AKA bondo, but why go thru all that labor when you can just swap out a door skin/frame for close to nothing like tedv metioned. IMO, that is the way to go, it's super easy and simple fix too! Being the fanatic I am, when I wax my cars, I wax the door sills, jambs, interior door skins and interior trunk lid.It's a super clean 850 but it's a PITA, but I want to make sure that at least those seams at least have some sort of protection inhibiting rust. Good luck on your project. Sounds like a fun one!