Alright, here's my attempt to fix United (long post coming).
Amorim - the manager has to go first and foremost. Not a single player in that squad is improving, in fact the development of Garnacho and Mainoo has gone backwards. That alone is enough to warrant sacking, add in finishing 16/17th, picking Shaw and Mount for a European final - he's shown no tactical flexibility or pragmatism to get the best out of the talent in that squad and rewarded mediocrity, he simply has to go.
Bruno - get rid of the petulant fool. Don't care if he's the best player, he's an Amorim loyalist and he's the leader that has set the standards as low as they are now. Take whatever Saudi will pay and reinvest it elsewhere.
I am on the fence about Maguire. I think he could stick around if its an English manager coming in and someone with experience needs to be captain. If the new manager is coming from abroad then get rid of Maguire. I wouldn't even let the new manager decide if he keeps Shaw or not, that guy has to be cut loose.
Those are the big chess moves, I would hear offers for any other player in that squad not named Mainoo, Dalot, Yoro, Garnacho, Zirkzee, Dorgu, Ugarte, or Diallo. There is some chance they could be good enough players under another manager.
Now we get to recruitment. I'm setting up for some variation of a 433, with Mainoo and Ugarte there's the option to play either a double pivot or a 6 with two 8's. My priorities are a GK, CB and a CM. I'll assume Cunha is already done. There's enough of a back 4 there already with Dorgu, Mazraoui, Yoro, Martinez and Dalot. If anyone wants De Ligt they can have him, his wages are too high for a back up player but I suspect he won't want to leave.
Pending what Maguire does, I'd approach Palace for Guehi. His contract is running down and Palace are ready to sell. Offer £40-50m and get it done. His experience and leadership would be invaluable to this squad and the development of Yoro, might take silly wages to convince him but it's worth it. Give him the captain's armband on the way in.
Looking for a CM slightly more offensive minded to replace the creativity of Bruno, I'd see what Brentford want for Damsgaard. I think that could get done for £50m, maybe offer them Rashford or Antony in return to sweeten the deal if they're asking for big money. Unfortunately Mount won't leave with his wages so he could just be depth for the role.
GK is tough. I'd be looking for value and for PL experience. Ramsdale will be on the move, likely Kepa too. Both those guys would jump at the chance to start for a big club - see which one is the cheapest and make it happen, £20mil would be reasonable.
That's £120m + 60m for Cunha, takes the spend to £180m - most of which should be off-set by the sells.
Leaves an XI (+ depth) of something like:
GK : Ramsdale/Kepa (Bayindir)
LB: Dorgu (Amass)
LCB: Guehi (De Ligt)
RCB: Yoro (Martinez)
RB: Dalot (Mazraoui)
DM: Ugarte, Mainoo (Collyer, Kone)
AM: Damsgaard (Mount)
LW: Garnacho
ST: Cunha (Zirkzee)
RW: Diallo
Anyone not named above is sold to the highest bidder. Likely the squad is short one versatile winger, pending how much money is left over I'd use that for Mbuemo or just hang on to Sancho, use Cunha occasionally on the left and fix it next summer.