just drive it into the ground man. avoid potholes, and jumping rr tracks. I've seen a few cars that have some severe rust, that if you look at them you think they aren't safe, but some crazy stuff goes down in them and they still hold together. In fact, I own one of them. So, drive it for the time being, as-is. Maybe keep it under 70, never a bad idea. And once you find a good deal on a car, turn this one around cheap to someone else. I wouldn't mess with it, because sometimes rust is best left alone.
'89 SSE - GX3 -190k, 155k on body - Back for another ride.