If Grealish and Harry Kane are both transferred to Manchester City, Manchester City will have sufficiently strengthened their squad. They managed to keep the squad from last year mainly.
They will seem to have paid a very high transfer fee for these two transfers (about £230M) and I think they will give a lot in return with the trophies they will win.
No doubt, Harry Kane will definitely be of a great help if transfered to Manchester City. He had been the top target of Manchester City. And it seem he will be transferred to City. But, Tottenham placed £160m on the player, and I don't think the price will stop City from signing him. Since Aguero is no longer in Manchester City, they however need a good, talented striker to replace him.