Two arbors and two face mills if you plan on using both routinely, otherwise one arbor and two face mills. If you have a one piece face mill, if it gets damaged it is a total toss. Separate arbor gives more flexibility, most of my face mills I picked up over time at about 25% of list, eithe new or slightly used. I have several face mill heads (5) and use two arbors mounted to the two face mills that I use most frequently. Insert wise I have some heads setup for softer metals/materials, others for steel, etc. You can get heads/inserts that can be used on a wide range of materials, but you get better results with material specific inserts/heads.
Setup for softer metals with sharp ground edge inserts
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Steel/harder metals with molded type inserts
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