2002 3.5L Montero Limited gets hot & turns off?
 

Do these Montero have computer features that shuts them down when overheating? A 2002 I'm looking at got hot (temp gauge spiked to max) and then 10 minutes later shut off. It started back up a few days later and did same about 5 minutes into test run. It has full oil and coolant. Unknown if head gasket or other cause. Owner said it smoked some from tailpipe on both overheat occurrences.

That is a known issue with the instrument cluster. Search the forum and you'll find pretty good discussion on this including how to DIY repair it.
The PCM reads engine temp from a different sensor (on a different circuit), not the one you see in the cabin.

But do they have a shut off mechanism that turns off the vehicle when it gets too hot? The interior gauge is spiking upward and then shortly thereafter it turns off (so says the owner). Can only motor seize cause this or is there any other reason this could happen in conjunction with overheating? He said it did this once and a few days later they restarted the vehicle and it overheated again and turned off again.

Are there any other signs of overheating - puking coolant, engine being excessively hot under the hood, loss of power? It sounds like NO to me. That means there is a 99% chance it is just the temp gauge malfunction. Many Monteros develop an issue with the instrument cluster circuitry that falsely indicate high engine temp for a short time. It is scary as hell and is very annoying. To answer your question - I don't know if the engine will shut down on its own to prevent damage from actual overheating.

Yes, according to the owner's kid, it puts off some form of smoke or steam from motor compartment, as it's showing hot on the gauge, and then shuts itself off. This doesn't sound to be the cluster malfunction.

sounds like its time to check for head gasket integrity

Head gasket is blown right?
 

So I filled the radiator coolant up to brim, left the cap off, and started the Montero. Coolant erupted (about 1 cup a second) out of the radiator top.

This means definitely head gases blowing into the coolant passages right?

It wants to start and acts like it would but I think the plugs are maybe getting wet after a few seconds and then it dies and will just continuously crank but never start-up. Head gasket?

I don't hear any grinding, rubbing, clanking or screeching though.

I'd be worried about getting the engine hydrolocked if so much coolant might be getting into the cylinders. You can take out the plugs and turn the engine over to observe which of the cylinders might be blowing coolant of of the combustion chamber. That would confirm your head gasket issue.