Luke is only a top LB when he is not injured and in good form. Other than that he is just a good defender, who can play LB and LCB. He shouldn't be a clear starter and needs competition. Sure, we could gamble on Amass, but I would prefer to loan...
Between Dovbyk and Sesko I'd get the latter but the idea of getting the La Liga topscorer for 40m euros is really tempting (since reports say that Sesko's clause has risen).
If the reports are true, Thuram would be 15m, Fofana around 25m and Onana would be the most expensive at 60m. So definitely not 175m.
Edit: I'd go after Thuram, Zubimendi, Hackney and Gourna-Douath and sell / release Casemiro, Amrabat, Eriksen...
I personally would rather take Bruno because we need instant improvement over long term solution on that position. And I think Bruno G.'s got a release clause + NUFC have money problems, so he should be available.
INEOS would surely know if he is good for United or not. If he's for sale, I am pretty confident they will try to get him here, unless they don't rate him that much. For that amount you can't get better players.