Personally I would basically agree with you on all of that.
I don't really see anything too wrong with running the 600k KM head IF the engine it came off of was known to run well and didn't have any known issues with using oil or coolant, etc.
You would want to check that head to ensure it is straight with no warpage (straight edge), and probably want to check for any signs of valve face or seat pitting and valve guide wear. Look for severe cracks as well of course, although as you probably know, small cracks between the valve seats are normal and no concern.
Some people would also want to replace the valve stem seals although that is a more involved process. New camshaft seals could be wise and those are fairly easy to change. You can check valve guide condition by measuring lateral movement of the valves -- severe wear is easy to spot. You can also check valve sealing using water in the ports, and look for any signs of oil intrusion in the ports that would suggest leakage past the valve guides or stem seals. Or if you really wanted to take it all the way you could hand-lap or replace and lap the valves.
But if the engine made it to 600k and ran well it may be just fine to drop that head on with no work done to it, assuming it checks out. Valve guide wear used to be a problem on these engines but with good synthetic oils it is not much of an issue from what I understand now. And an engine that put on 600k km and still runs well probably has to be one that was maintained well and mostly saw highway usage. Apart from wear to the valves/guides/seats/seals, you're correct that there's really not much to wear out in a "tired" head and it would probably run with zero issues, as long as it's bolted on top of a healthy bottom end with good compression (and timed correctly, etc, etc).
On HG thickness, yes you can take measurements to select it based on piston protrusion from the block. The greenbook shows the method. But changing to a different head doesn't affect the thickness of gasket needed. So you can just use the same thickness that it has now.