Ki won't want to be a backup. Not to mention we do not need quantity, we need quality. Very little chance of buying 2 midfielders anyway.
I'd put all my resources in to buy Schneiderlin. I am convinced by what I saw of him in these 2 seasons. I'd help greatly if we want to play 4-2-3-1 and we have Coquelin and Schneiderlin as our base. They'd win any midfield battle and our attacking players can have more freedom to play. I think players like Pogba, Verratti, Martinez, Busquets, Gundogan is pretty unattainable for now so he would be my first choice.
With Schneiderlin, Coquelin, Ramsey, Wilshere, Rosicky, Cazorla as CM options we would have quantity and quality. Then add another marquee player in attack or goalie and call it a day.