Does your graphics card support DX12 ?
If not you should also buy one along with going for win 10 only for DX12.
Not sure about 4.8 but yes if DX12 is a requirement for your game then your players should also own a DX12 compatible card. Unless you kinda let them switch do DX11.
Most DirectX 11 cards should support DirectX 12, but currently if you want a build of UE4 that has DX12 support you have to contact Microsoft. It’s not likely to be introduced in the 4.8 update