Hi Jams54!
I am currently playing the third season of an online game with two friends (United, Tottenham, Arsenal).
First season was fantastic with league victory and ChL final. Won the second seasons Premier League as well but ChL ended in the first knock-out to Barcelona.
I started out by getting Timo Werner and putting him on a loan playing 30+ games the first season. Your team is well-balanced, so you do not need to improve any positions yet (I brought Luke Shaw, but actually regretted since he is too young to take over after Evra, and I ended up getting Leighton Baines next season). In the same transfer window I sold Anderson, Cleverly, Nani and Hernandez.
For CB I got Doria. He's a top player. Tried to sign Balanta but he went to Arsenal. Get one of these. I would probably try Balanta first.
For the wingers I spent almost 30 mil on El Shaarawy after the first season. Expensive, but he is the best on his position in-game - and as United you'll need the best. I actually had much success with RvP as an Inner Forward on AML the first season, since I play Rooney as a False 9. This leaves room for the players behind with RvP cutting inside and scoring lots of goals. Its not the best alternative, but when injuries occur you have to be creative

.
Wanyama has just joined the team. Playing 4-2-3-1 without any DMs, I like to have strong hard working CMs. Wanyama and Fellaini are a killer midfield, making them effective both with and without the ball. Train attacking set pieces for some parts of the season and you will also score lots of goal on corners too. (Vidic, Jones, Fellaini, Wanyama, Rooney in the box at corners). I play CMs as BWM and Box-to-Box by the way - none of them too creative. Both players can actually cover both positions with Carrick as a backup still. I also signed Obi Mikel as a backup, but this really depends on the transfer list. Always check for listed players - you can do a great deal sometimes.
Here are some players you may look at. I do not have the save game on this PC, so it is only a rough guess:
GK: Leno, Bardi, Romero
CB: Doria, Balanta, Bartra
LB: Jonathan Silva (cheap), Luke Shaw, Ben Davies, Leighton Baines, Alaba.
CM: Wanyama, Pogba, Verrati, Sergi Roberto,
Attacking midfields: Ocampos, El Sharaawy, Vitinho, Kovacic, Bernand, Kevin Volland, Julian Brandt, Shaqiri (so many choices on this position)
ST: Timo Werner
Good luck with your transfer window.