'85 245 GLT - how much rust is too much?

Howdy! Have been getting into fixing my old Volvo recently. Had a long
illness that kept me from maintaining her well and she's worse the wear for
it. Amazed that the car still runs with all the years of neglect and abuse,
but am having fun learning and fixing and the car's responding well to the
repairs. Have been replacing essential engine components to get her running
well again. After 230k (estimated - odometer broke at 187k years ago), a new
exhaust system and a tune-up has her purring like she did when I drove her
for the first time (well, except for the vacuum leaks that I'm going to fix
this weekend). Am putting in a new wiring harness and new suspension
components (bushings, shocks, springs) to make the ride nice again, too. It
dawned on me recently that the brown holes in the car were kind of important
to learn about. Don't have any experience with rust, but figure it's
something I should probably pay attention to before I sink scads of dough
into the restoration of my trusty, reliable work-horse. There's some rust on
the rear quarter panel, below the driver and passenger doors, around the
windshield and some small spots on the roof. The areas below the doors are
large - 3-4 inch holes. The wheel wells are clean and rust-free. The
undercarriage looks good, although some components have lost their paint and
have a nice orange coat on them. I've cleaned and painted a few parts and
will either do the same for others or replace them, but the body has me kind
of nervous. Any advice or input appreciated. No matter what, I'll do the
minimum required to keep the structure sound and air-tight and drive it till
it falls apart.

Mike