2002 Ford Taurus head gasket needs replacing?

(Car had one previous owner) I discovered that the fluid in the radiator was like muddy river water and the reservoir had obviously overflowed, probably numerous times. I had it flushed, then began noticing fluid on the ground under the reservoir area and the engine began overheating (previously it had only overheated once, about a yr earlier). It's just been a few days since it worsened and I got the diagnosis, and I need to do something quick. I only go short distances and keep containers of water handy for when it gets hot. Also, after driving for around 10 minutes, I can feel and smell warmth coming from the dashboard area or under it.

Q: My mechanic is 98% sure it needs a head gasket repair. I really trust my mechanic for various reasons, but what are the odds that it's a different problem?

Q: approximately how much would this cost, assuming that's the only problem that's found?

The car body and interior are in very good shape, and overall, I'm really pleased with it. Plus, I just paid it off in April. When I bought it I was able to put several thousand down and made $225 payments for 3 1/2 yrs. My financial situation now is such that I MIGHT be able to scrounge $1000 for a down payment, and that would include the approximate $400 I'd get for trade in.

Q: is it worth making the repair rather than trading it in/scrapping it?

After researching a bit, I'm finding that I'd be lucky to get a car for $4-5000, and that's if I can come up with an acceptable down payment. My only income is SSDI, so a really decent down payment would take all of 2 month's income, which obviously, isn't an option.

Q: I've heard a lot of both good and bad regarding carmax - your thoughts, if you're allowed to address it?