Head gasket failure

[secondchance]

Yesterday as I drove home from work I noticed my Low Coolant light was coming on and off. So after parking I filled the over-flow tank and went to bed. This morning when I started the car the low coolant light came on again. I checked the over-flow reservoir which was ok then opened the expansion tank - about 2/3 gallon low. I filled the tank with distilled water and started the motor. As I drove out of the parking I noticed white vapor coming out of the car - steam.
It seems I have a blown head gasket. I called a shop I dealt with for 3 years for servicing my BMW (the owner has a 90 ZR-1 and services Ferraries, Astons, Porches, etc...) and asked for an estimate.
Now that the heads will be coming off, any suggestions of services I should have done during this work? A proper cam timing, R & R cam guides, valve job, etc...?
Also, what has me puzzled is that the car never overheated. Do head gaskets fail due to age and mileage? Would you suspect use of non-GM speced coolant? I have been running Preston Extended Service coolant. Any thoughts?