Upgrading 802.11n to 802.11ac
The Apple developers forum confirms that the new Auto Unlock with Apple Watch feature offered in MacOS Sierra works only with an 802.11ac, not with an 802.11n.
So if you want this feature to work, you will have to replace your <2012 n with an ac board.
The question then is the part number that corresponds to the 802.11ac that works in a <2012 MBM.
Yes. Mac Mini.( And some more characters to meet the minimum requirement.)