Need advisement on repairing my heater core. Recently I took my 1990 BMW 735iL to my mechanic for an look-see and was told that my heater core needs to be replaced. I knew that just wanted confirmation! Big Bucks! $$$..they want.

My plan is to do the repair myself, but my 1st choice is to try the quick and easy fix by trying a coolant sealant like "Blue Devil" ($60 Bucks) to see if that will work. Can anyone give me some advise on whelter these products work and should I try it on my car?