Um, well at 180K a lot of "maintenance you will have to do" depends mightily on what's already been done.
I would check the wiki for info.
I would also contact a BMW specialist mechanic and arrange a thorough inspection of the car, unless you are knowledgeable enough to do one yourself.
For starters, you could ask when the last time the following services were done:
- Cooling system (complete: radiator, thermostat, water pump, expansion tank)
- Fuel filter
- Fluid change (engine oil, transmission fluid (AT or MT), differential fluid)
- Braked components (disks, pads, fluid)
- Control arms, bushings, struts, rear shocks
If you get a "Uh, uh..." for any of the above, ask for service records. Poor service coupled with this mileage is not going to end well for the purchaser.
I would allow about $2,000 in annual maintenance costs for the car.