Excitement aside, where will we actually fit him in? I don't think he's starting ahead of Casemiro and a Casemiro - Rice pairing is not what we want given our interest in Mount.
He seems tailor made for Arsenal if Partey leaves. Rice + Lavia is going to be amazing for them. Or if they go the same route as the other top clubs with two interiors (Havertz, Odegaard) protected by one DM (Rice) that would work too.
Yes as you suggest in your second paragraph. If you change your system and push up your lines, Casemiro will very likely be benched by a Rice or Caicedo at six. Its not a contest.
Casemiro is not a player who cannot patrol space better than Rodri, Partey, Caicedo, Rice or even Guimaraes. He is a better fit for a deeper lying team as United were in 22/23 and is excellent at anticipation and breaking up opposition attacks.
A cheaper, more rounded option than Rice or Caicedo, would be Bruno Guimaraes IMO. He will also bench Casemiro in a more attacking system like City's. But more importantly Guimaraes can play the defensive setup midfielder role better than Casemiro as well, as he's able to hold the ball well, break up play and advance play as well.
As for Mount, he resembles a box-box player to me as well. Creative, defensive, and comfortable with both feet and hence can cover the defensive and attacking midfield lines in your team. He would be a good signing for United, absolutely.
For United to do business quickly, I'd say get in Mount, Guimaraes and Rabiot. That would cost 100m but it would give United flexibility and depth in central areas. Fernandes, Mount, Guimaraes as your starting three, with Lisandro moving into midfield in possession. Skillful, energetic but also a solid defensive base for the front attackers.