Wow, you must live at the gas pump doing that mileage a year on a 50i
Tough call, but I'd have trouble spending 7-9K on a vehicle that is only worth about 15-18K right now. I'd almost just drive it until it becomes obvious that is needs replacement. Even with a replacement motor it's value is going to decline further over the next 2-3 years, there's just so few people that want to get into these vehicles, especially a real high mileage one.
Just my .02.