The United squad is already good enough - in fact you could probably do with selling a couple of midfielders, bring in a bit more cash.
Because the squad already has great depth, and you get a huge transfer budget, maybe a big-money signing (Balotelli (around £25mil)) or Jovetic.
United are weakest on the wings, and Balotelli and Jovetic can both play on the wing and are young.
Aside from that, buy Otamendi. Will become a world-class centre back.
Sell as much excess as you can in the first year, depending on the game defined PA perhaps both of Welbeck and Macheda for 10 million EACH. Park tends to not be very useful when you can harness the Workrate of Valencia. Get rid of every excess player that is not in the youth team, including the likes of Gibson, and you might want to consider selling Tosic.
Then purchase Douglas Costa for around 6 Million from Gremio and Marquinhos from Internacional for a similar price. This will put you deep in the positive for transfers which is great, but most importantly give you a trully awesome young winger that not only has pace and dribbling, but intelligence, creativity and goal scoring abilities. It will also give you a slightly slower but highly creative young forward/AMC that can fit into a multitude of tactics, but also replace Berbatov over the coming seasons with a much superior "mentality" creative forward.
The final real key detail is to then retrain Jonny Evans as a RB. He will never be able to replace Ferdinand or Vidic but he can fill in for them, and he will easilly surpass both O'Shea and Brown at rightback in almost all contexts. Rafael could then be retrained as your next Right Winger if the players you currently have down the right don't cut the mustard.
Half way through season 1 and definately in season 2 you will receive an influx of players back from loan, promoted from youth, or recovering from long term injuries (Hargreaves). Do not damage yourself by failing to be ruthless and sell as many of these players including underperforming RW's in order to purchase a trully world class right midfielder while achieving a positive balance. With sufficient ruthlessness you could buy Aguero outright and still end up with a positive balance.
Rooney, Berbatov, Owen and Marquinhos upfront, combined to Aguero, Rafael, Nani down the right, with Giggs, Nani, Costa down the left, with Fletcher and Carrick and Anderson and Scholes though the middle, with Brown and Evans and Rafael down the right and Evra and Fabio and O'Shea down the Left, and Vidic and Evans and Rio and Brown and O'Shea through the middle.
A world class team of epic transfers and amazing young players that is still operating on a positive yearly balance.
For season 3 you must focus on a replacement goalkeeper. Adler remains pretty much the best for United but you will have to splash some cash so you will have to sell some players.
Canales can be got an a bosman for free halfway through the first season. Snap up as many of these as you can even if for the sole purpose of selling them for top dollar. This is the managerial life you now lead. A strange combination between LLM and European Cup finalists.
NEVER spend more than you can make in any season. Most seasons you have to sell more than you buy but you can do this with the excellent youth system United have.
If you must dip into the misleading transfer kitty then do so only to buy one player that will be world class for years to come and you absolutely need.
Right Winger and Goalkeeper in the early years.
I would advise against Balotelli personally. You cannot afford to splash bucks on players that might not become top notch. If you are going to spend 20+ million on Balotelli you might aswell go whole hog and buy Aguero.
The real challenge is to try to give United an English face, i can´t stand if my side (doesn`t matter which one) does not at least field 5 homegrown players. United with 11 foreign players is just Arsenal to me. If you put that sort of restrictions on you it will become considerable harder to be successful.