Is it a 3.0 or a 4.4?

First off, you should be using BMW coolant and not Prestone. I'd also be getting a second opinion as a blown head gasket should have definitely had symptoms such as sludge in the oil...unless its a very minor leak, in which case a new head or block should not be needed...anyway, I would definitely get another opinion, or even two, before either spending $10K or sourcing a junkyard motor.

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BMWCCA Chapter President, Hawaii Chapter (fmr)
03' X5 4.4i, 02' MCoupe, 99' MCoupe (sold), 88' M3, 01' E39 M5