If they got Casemiro and De Jong, they'd probably be top 4 contenders with Arsenal, Spurs and Chelsea.
It would be a very good start.
But they're not going to. They'll get Casemiro, think job done, then pair him with McTominay and Fred and it's a half measure.
I think United actually need to do the opposite to what you're suggesting. They don't need to build a team over 3 years, they need to replace 8 players in one window to completely reset the culture. What they've done for the last 5 years is bring in one good player a season, and they get brought down and corrupted by the rest of the group by the time the next guy arrives.
Also they won't be able to do anything with De Gea as the keeper. He just can't play out from the back, so until he's gone they're treading water at best.