On a scale of 1-10, it's an 11. You'll need to have the head checked by a machine shop for flatness and/or cracks. Depending how bad it was, it may have bent valves or scored cylinder walls. On top of that, you'll need all of the cam timing tools which can get pretty expensive. Parts and special tools will run you about 400-500 if you're lucky and don't need to replace the head. Add in machine shop fees and about 20 hours of labor. Most shops will charge anywhere from 1000-2000 for a head gasket job. If I were to pay a shop to do it, I'd only take it to a BMW specialist who warranties their work, because if they mess it up, you'll most certainly end up doing it again soon. There's a whole bunch of stuff that you should do while you're in there, most notably replace the plastic cooling system parts, water pump, thermostat, radiator, hoses etc. For the cost and time spent on doing a head gasket job, your money is probably better spent towards a 6cyl swap. | 
Buying my second TI but have a ? 
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