Best players to United as per my experience: Erik Lamela, Sharawy, LEandro Damiao* (Beast), Cavani (lasts till his age of 29 everytime in my game), try to grab ayew, yormalenko, adryan,Mateo Kovacic, Alaba as early as possible.
Sell: Welbeck, Chicharito, Nani, ashley young, anderson, RVP as soon as possible because they will not help you much.
areas to be strenghten are Full backs, you will get Baines chieply, and for some seasons I kept NAthanian clyne. with Raphael, I sold Rapha for 40-45 mil to MAdrid.
Players do not sell : Jones, Fletcher, Carrick, Vidic, Smalling, Evans as they can produce goals and an avarege of 7.70-7.90 even after they passed their picks. Keep the training to attcking set pieces and see the magic of Vidic for some seasons.
Sell Rooney after 29 as he turns goalless and a burden to the team eating a hefty weekly salary and will give you headache.
Next GEn players : Januzaj, send him to loan for 2-3 seasons, and he will be an asset at att Midfield.
Players do not try: Rodrigo, Bernerd.
** Try to collect some good regens having potential to 4* in their earlier days.