To be honest, Nkunku isn't the first player that comes to my mind when thinking of a potential ETH line up at United. I rate Rangnick but let's not forget he comes from a very different school of football than ETH. Nkunku excelled in systems that are very different to the Ajax one, so it's not really clear where he would play. It could be as a false 9, as a winger or as an 8, I'd say.
Generally, I believe Sancho and Nkunku have enough quality to run the show for a top 4 team in the EPL, yes. I believe in a cohesive system Sancho can become just as good in the EPL as he was in the Bundesliga - he's so underrated right now. Bruno Fernandes is a very difficult topic, though. He has good assist and goal stats but I think they come to the detriment of his team since he is so wasteful with possession. I believe a coach like ETH will think more about the "opportunity costs" of his plays (how many own chances could've been created and how many opponent chances prevented if he hadn't gifted the opponent so many easy turnovers?). I've also read a few times that his pressing behavior isn't very good. One way or another, Bruno would have to become much more tactically disciplined under ETH. His teams operate highly tactcal and Bruno is rather instinctive in his movement and decision making. I don't think it is a given that he'll play a huge role in United's future. On a sidenote, I think he'd compete with Nkunku for a spot.
Elanga is also difficult to tell since I haven't seen that much of him. But in general, I believe that cohesiveness beats quality, especially if you have the latter in other positions. So if Elanga is hungry and willing to learn, I can imagine that he's part of a line up able to get a top 4 finish at least as a complementary player. Personally, I'd still think getting Antony is a no brainer though. I'd rather go for him than for Nkunku if I had to choose. There aren't many left footed wingers around these days and he's incredibly talented. Not that Nkunku isn't as well but you'll have it easier to find a substitute for Nkunku on the market than for Antony - on top of having no player in your squad who really excels at RW in contrast to your direct competitors City and Liverpool.