OP, 80k miles per year...I'm going to state the obvious but that's a lot of driving. In general, the components on the M3 are going to be more expensive to replace plus average MPG will be less. From reading post of owners with high miles, say north of 50k, there doesn't seem to be any major areas of concern. But whether a standard or extended warranty, you would go through it within 15 months (give or take). What makes the M3 so appealing is that it is a great DD. However, how will the car be after 2-3 years and 160-240k miles, I don't believe there's enough data to know for certain.
For maintenance, fluid changes including the diff, gear, motor, brake and cooling. Additional will be whatever breaks.
If you are keeping the M3 for one year, except for the vehicle depreciation, you should be okay.