I'd ask what type, and size of engine. On cylinder heads, maybe engine blocks too, are they cast iron, or will you see aluminum too ?
In years past we built all our own cylinder heads, but we're talking industrial and marine diesel engines that would see many overalls in they're life. These days manufacturers offer core exchanges that is difficult to beat in cost with warranty coverage as well.
I don't recall the brand name of the machine, but it was not a mill. We used a resurfacing machine that used a big ass grinding wheel, and lower half of machine was basically a coolant tank. Took up pretty good amount of floor space as well.
I have seen heads and manifolds resurfaced on a mill using a fly cutter. Could be operators did terrible work, but I don't like the finish from the fly cutter. The finish spiral lined and could be felt with my hands and finger nail. Maybe problematic for fire rings on a head gasket.
I'd probably set up with a face mill cutter if I was going to do it. Maybe a No 2 vertical Cincinnati or the like. Or a horizontal with vertical head. But I'd be using the vertical for resurfacing heads and manifolds. A no 2 size has good weight and power without being too monstrous. If your electric service and floor space can handle it, maybe a No 3.
But maybe a Horizontal with vertical head is the best choice. Depending on height of engine block, might not be able to get an engine block under the vertical head, with block upright. But flipped on its side, you could put a face cutter in the horizontal spindle. . . Food for thought anyway.