My head cracked about 2 months into owning my 'is. It was fall when it happened, but my symptoms were:
Losing coolant
Lack of heat with heater on
White smoke from exhaust under throttle
Note that mine leaked into the engine, but it can also leak out.
I shut it down at the first sign of smoke, found a guy locally that specialized in bmw's, and ended up replacing the head with one that had been resurfaced, and had a valve job. Total cost was $1700.
If you do the job yourself, inspect the head VERY carefully. The crack in mine was TINY!
As far as parts, you'll need:
Head gasket
Intake Gaskets (upper and lower)
Valve cover gasket
Timing gaskets
Thermostat
Thermostat housing gasket
Radiator Hoses
Head bolts (or studs if you want to upgrade)
Cam locking tool would be nice to have
Also a good time to tackle timing components
Timing chain
Guides
Tensioner
Cam sprokets
And, if you haven't done it already, bypass the heater plate on the throttle body take a look
here