I think Harry Maguire is a decent signing for them, it does sound like a lot of money though for a defender that I wouldn't consider top class, he's great in the air but suspect on the ground and is vulnerable to being dribbled past - I've seen Mo Salah tear him a new one. The question for me about not just this signing, but all of them, and I've said this to a few Manc fans is what style of football is the team going to play? As already pointed out, take Maguire and if they decide to play with a high line with him in the team then they are likely to face problems. He'll be OK in a set up that involves playing deep and hitting the opposition on the break a per the successful Leicester team from a couple of years ago, but somehow I just cannot see that style of play being popular with the fans at Old Trafford - look at the Mourinho teams for example.
In terms of alternatives for them, I get the talk about Koulibaly, a good defender with pace, physically strong and good in the air. I don't quite get the hype about De Ligt, he looks a good prospect but lacks physical presence an I've seen him bullied out of games, for example by Llorente when they played Spurs. He could improve in that area but for defenders who are renowned for being dominant in the air, it tends to be a natural attribute rather than coached into them. So that side will be a big plus for them with Macguire, its been our achilles heel for years and it is no coincidence that we have started winning trophies again since adding a top quality CB who is dominant in the air.