To your question of milling the head, there was a thread here concerning that.
Essentially there are "holes" that are close to the sides of the sealing surfaces of the head. Depending upon how much "margin" based upon depth in those holes that is remaining, that is the area where you measure remaining meat at multiple spots.
You can only go so far, before the manufacturer considers the head to have too little clearance for factory parts, and barring special thicker gaskets, there would be no coming back from.
A certain amount of warping could possibly put you outside the margin as well, but for a car running properly, I'd say the odds of it being so far off to be non-repairable or the core unusable are very unlikely.
Just be sure to remove the head bolts in a reverse-to-installation fashion working outside in, which is opposite to inside out for installation, and turn each bolt a quarter turn in passes.
This way you can help the core to be more likely to stay in circulation. I speculate on that last part, but it stands to reason that the less you stress a part coming out, the better it may stay.
Also, consider keeping the head that comes off. You never know if you might need it and it costs minimal money to have it disassembled and checked. Heck you can have it rebuilt by a reputable shop and keep it around.
I like the idea of a built head as mentioned by previous posting, but it all depends upon your goals.