This is just an okay car. I wouldn't be concerned with Nav/iDrive as the 08's have the old system which isn't even close to the new one.
I think you can get a dealer CPO'd 08 e92 for low 50's (based on what I bought in September). It's the dead of winter so check to find a dealer who has had an M3 sitting on lot for a couple of months - then negotiate. If it is listed on bmw.ca's used inventory at $59,900 you can get it for $55,000 out the door in most cases (again based on my buying experience). If the car is CPO'd this will cover you for the "grenading engine or transmission" comment you made.
To summarize - if you love the one at Legendary, buy it. If it's not everything you want compile the list of 08 M3s from bmw.ca and start making phone calls/dealer visits.
Your negotiating ammo is:
1. January is the worst month for car sales
2. It's f"n freezing and an M3 is a tough sell in the winter
3. New F30 is coming
4. You have already found one for <$50K at a dealer (not private seller)
5. You aren't bound by geography - shipping a car is only $1,000 cross country
Good luck and post some pics when you find one