Here are my suggestions based on a few saves as Arsenal and Bayern
Established stars (if you want to go for that instant star "wow" factor):
CM/DM: Paul Pogba - Available for around 50-90M quid. Huge fee and expect high wages and huge sign-on and agent fees. BUT he's the best CM/DM in the game, a huge star and starts off World Class (or becomes it very quickly).
LB/LM David Alaba - Available usualy for around 30-60M from Bayern. One of the best LB's in the world and especially as Man Utd where you want to take the game to your opponent he's extremely adept at this part of the game. Overlapping and rushing forward.
CB: Eder Alvarez Balanta - Available for 5-10M. Not a star at the moment you say? Well, he should be. Got a fixed potential that is World Class and he's probably the best CB in the game. Got stats for a CB in all areas and is a must have for your central defense.
Young World Class potential players:
AML/AMC/ST: Memphis Depay - Availible for around 12-30M quid. Extremely high potential. Won tons of awars for my Arsenal side in third season, including World Player of The Year, CL Top Scorer, English PL Top Scorer, Player of The Year, Player's Player of The Year and scored 38 goals in the PL and 16 in the CL.
CM/DM: Lucas Romero - 5-10M. With the right training and playing time can get to Pogba's level. Extremely good CM/DM defensive stats, great ball winning player to get hold of the ball.
LB: Jonathan Silva - 7-12M. Not high reputation in the beginning but as far as I can tell he almost always becomes world class.