I've never gone into my engine so I can't help you, but you'll want to figure out why the car overheated. there's two plastic fitting notorious for breaking, one's under the manifold and one's on the back of the head, make sure they're not broken. make sure the thermostat's not stuck closed and that nothing else in the system is broken. if all looks good it's likely the PO did some work on the cooling system and never bled it correctly, which is fairly complicated on these cars.
here's an excellent write-up on bleeding the cooling system
http://www.318ti.org/forum/showthread.php?t=27983 good luck, hopefully you were lucky enough to just blow the head gasket, but if you drove it overheating for an extended period of time, you may have warped the head
