Going to look at a 98 Tahoe. Owner claims he found coolant in the oil and had it verified by mechanic that the intake gaskets are shot-obviously a common issue with these engines.
What I want to know is, can/should the engine be started with this going on? I'd like to verify its not head gasket, etc. If so, how long could it safely run so I can check things out or possibly move the thing a little?

Highly doubt its the head gaskets. Based on the history of these type of GM V engines, the gaskets suck in them.

Replace the gaskets, change the oil. Done. You wait any longer, you're looking at a rebuild. No joke, been there, done that.

most likely fine, mine looked like it was run that way for a while. need to set the cam retard after and pay attention t the torque values, inch lbs gets alot of people to warm the plastic intake, and took me about 2 hours to clean the engine, rockers and junk from all the white stuff