The head doesn't rust at all. But it will oxidize and with the dissimilar metals of the block and head, electrolisis will occur. That is why you must always use a good antifreeze and have ground wires running from head to block and head and block to body.
I have seen what you have been described to on quite a few heads. They can be fixed, but a lot of heliarc welding to fix the ports and in the long run a good used head is a better way to go. It seems that you just can't get the head gasket to seal around those water ports and the combustion area and it ends up leaking antifreeze into the system somewhere.
Try to find one at the junkyard or look on ebay, just make sure you check the water jacket ports on the bottom of the head first or make sure there are good, clear pictures of that area if you bid on something.
I just pulled the head off my turbo supra to fix a headgasket. I have another head I am going to use, so if you can't find one let me know and we can talk about it.
Russ.
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