Forget the sealer. Even IF it worked for a short time, it won’t be reliable, which you said you need. They’re correct about the diagnosis, it’s normally a head gasket, but it could be a cracked block, or a cracked head. None of them are pretty, but the head gasket is the easiest to repair.
Judging from the price you list for the head gasket, this sounds like a dealer. Chat with some friends, and see who they use to maintain their cars. Hopefully some will be using a local mechanic. Find out who it is, and grab the phone, and call. Ask about a replacement or a good used engine. You may be surprised.
Chase