What I think is going on is, your engine overheated and blew the headgasket. That means, there's NO WAY it's flat and straight. So it would have to be machined.
The "donor" head is coming from a good engine, not overheated, and in theory anyway, no warping or out-of-true problems with it. The machinist can check that, after removal and cleaning. Spec for this is only like 0.006 inch (6 thousandths). It needs to be pretty straight to avoid machining, in other words. You MAY be able to get away with not machining the new head.
You always CAN ask that the new head be milled, but obviously such work costs money and time, and some people need to save one or both things when doing such work.

Typically, BTW, the used heads go for about $150. Seems to be the going rate but sometimes are available much cheaper.

FWIW, I just pulled the head off this 84 242 I bought, with a known blown headgasket. The engine was running, but very very sick. It had been overheated BADLY. Found missing metal between the valves on the #1 cylinder. A big crack or groove almost 1/8 inch X 2 inch of missing aluminum. Kinda scary. It'll simply be replaced, no fixing that without extensive welding and machine work.
Good luck with it!
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Rob Bareiss, New London CT ::: Roterande Fläkt Och Drivremmar!