Let's forget about the financial situation faced by Manchester United. Let's assume Manchester United successfully signed the players they want after letting go of their old players. Can they stay up to a season or two without having a good performance? I am asking this because it took Chelsea more than a season before their performance began to improve the way it is currently after changing coaches.
Manchester United is no doubt in need of a new set of players for the progress of the team. All that will take time before the players begin to know each other's playing patterns.
Manchester United will have enough money to buy new players if they let current players leave because it will be tough to get money for top players in the market right now if they still don't sell their worst signings. You're right, we don't know how long it will take Amorim to build this team if they sign more players, because it took Chelsea more than two seasons to improve, Chelsea finished below 10 positions last 2 seasons and managed to finish 6th under Pochettino because the players did not have a single coach for a whole season.
Manchester United management trusts Amorim to build the team, but they are still looking for players who can improve their performance. Perhaps Manchester United is only interested in Premier League players, as they will have no problem adapting to the league when they join the team, however signing a player from another league might be very difficult to get used to Premier League. I hope they can sign Mbuemo from Brentford too.