Hopefully, I can help. Cars are my life, literally. First, I'd say a 2008 Highlander with only 100k miles on it is barely broken in. Those things are good for 250k+ miles easily without needing anything truly major. Sure, things can happen, but the chances are slim with this vehicle. Remember, the Highlander is essentially a Toyota Camry station wagon....they share the same platform, powertrain, etc... Unbreakable car.
I would also very highly question the mechanic who gave you that quote. Head gasket? Are you seeing any white smoke out your tailpipe? Have someone check your coolant and/or oil. Do either of them look like a chocolate milkshake? If no, then you probably don't have a head gasket issue. Those would be very uncommon issues for a Camry/Highlander. A valve cover gasket seeping...sure, maybe. Head gasket? I won't say impossible, because it can happen, but unlikely. And if it's just "seeping", not the end of the world. All cars use gaskets. Over time, they can start to rot and they'll seep a bit. If it's just a little seepage, just drive it. If it's truly leaking, fix it. Whatever you do, DO NOT put blue devil or other "stop leak" stuff in your oil. It will create more problems in the long run than it solves.
Timing belt/chain covers can seep oil too, again, not really a big deal. If it has to be replaced, it's not horribly expensive. Water pumps are wear items, all cars need them replaced eventually. 100k is a bit early, you should be able to go to 150k.
I'd also suggest getting a quote from an independent mechanic, it'll cost you a ton less.
If you want a new car because you WANT one, then that's a personal decision and no arguing that. But financially, I'd say your car is far, far away from reaching that point.