Real Madrid have Eder Militao and David Alaba as centre-backs now and they are the starting eleven players actually. But Ancelotti didn't prefer Alaba in the starting eleven yet. Nacho Fernandez filled his place. And they would really make use of a top centre-back actually. Because it seems like they don't seem to be having problems on scoring. But they are conceding goals very easily.
Real Madrid have to focus on their defense system because new players, new team mates will not be able to play smoothly with each other, worse than players who play together in many years.
The average age of Real Madrid squad is high and it can be their problems in second part of the season, after the Winter, or when the Champions League goes to quarter or semi final. If Real Madrid can go far in Champions League, they can have troubles with the old squad.
But I agree with you that instead of Mbappe for 200 million, it is better to buy two good players in defense, in the center and on the right flank. And it was better to leave Hakimi in Madrid than to sell him to Inter.
It is not the tradition of Real Madrid and not the style of Perez. He wants to buy super stars and wants to build his Galaticos, from 1.0, 2.0 and now he is starting with Mbappe for the Galaticos 3.0. With his experience, I think Perez will not forget the importance of defense system so he will bring a few good defenders and defensive midfielders for the club. They will be parts of Galaticos 3.0