1997 Pontiac Transport Fix this Car?

I took my car in for repair because the check engine light was coming on sporadically, I had no heat (a/c works fine), and engine would become hot in stop and go traffic. The shop called me and told me that my car needed $8500 worth of work and I should just come get it. Here's the breakdown of what they reported: 1) Cracked head gasket. There was coolant in the engine, so they said that I probably have a cracked head gasket (which also explains the lack of heat). Their estimate for repair: $3500. 2) They said that the computer code for the check engine light indicated that the trasmission needs to be rebuilt if the transmission fluid is brown, which mine apparently is. 3) Brakes need to be rebuilt. They said that my bearings are bad, but that my brakes are still currently safe to drive with. Here are my actual symptoms. As I said, no interior heat, and the engine temp does rise in stop and go traffic. Engine temp is fine at any speed over 10 mph, and if I keep the a/c on in stop and go traffic, the engine temp stays low then as well. I've heard other people refer to having head gaskets repaired for $1,000 (for a sedan) and $2,000 (for a pick-up). Should my van cost that much more? As for the transmission, it seems fine. No hitches when I'm driving, accelerating, or decelerating. Mostly, when the check engine light is on, I feel no symptoms, but recently, for a few days, when I'd start the car in the morning, the CE light would flash for about a minute, and the engine would run rough. Then it would smooth out, and the CE light would stay lit. By the end of the day, the CE light would go off. I don't know about cars, but doesn't really seem like a transmission issue to me, maybe more like a clogged filter or something(?). Right now, the car practically purrs when I drive it, as long as I keep the engine temp low as described above, so it's hard for me to see that it needs so much work. How can I prioritize what needs to be done, and whether it's even worth putting money into this car?