Sometimes it is just the obvious things that need replacing. Reminds me of my ford. I was sitting idling at a light in 100 degree heat and the car started shaking. Ignition module went out and the heat was separating the electrical connection. Luckily I was able to put 2+2 together.
It doesn't sound like you have the experience, but I would be tempted to do a head gasket repair if I could prove coolant was leaking. I would get something to pressurize the coolant system to make sure there is truly a leak.
Wow, that looks nice. Yukon, OK here. Can I ask where you shopped? I'm looking to get something probably older. I think my budget is $5000-$7000. Maybe I need to save more?