Cool, kind of reminds me of my first car, a 1981 Chevy Citation (probably because they're both X-bodies). Do you know what size engine is in your car?
Hopefully, it just needs to be recharged, or it will be fairly expensive to repair. Did your grandpa ever switch it to use R-134a?
When I was barely 17 and beginning to driving my Citation, my insurance per year was $465 through USAA (for only liability). It seems like yours shouldn't be too expensive, either.
Keep in mind though that us folks with USAA are spoiled with our rates and most people won't have access to them.