If you have the mad welding skills, this can be repaired. If not, you're looking at a HUGE repair bill. One of the issues here is that the A-pillar is very much structurally important for the vehicle. This area really needs to be done correctly so it will retain it's structural integrity. The glass would need to come out (obviously). You also need to think about what else you'd find which has rusted away inside ... the part you can't see. Considering where it's at and the condition it's in, you can pretty much assume there's been water pouring down the A-pillar when it rains, every time it rains. I'm betting there's a huge amount you can't see.
My opinion? If you don't have the skills to do the work, pass. That is, unless you got some deep pockets.