I tend to like the EM12K wires unless you are looking for low temperature impact results and then you may have to go the higher manganese content wire.
You did not state what impact requirements you needed so the flux question will depend upon that and the ultimate tensile strength that you are looking for. Oh, one more thing to think about. Is your weldment going to be postweld heat treated? If so that may change the wire and flux combination that you select. I would suggest you talk your welding supplier and see what they have to recommend. We have used a lot of Lincoln products in the past but at present we are using ESAB. The type of welding equipment, how the process is being used, operator appeal and the final appearance of the weld will all have to be taken into consideration for the final wire/flux combination.