There's always going to be a dilemma:
1. You have to choose two from Keane, Robson and Scholes (although the more I watch of him, the more I'm thinking that Robson should be the first name on the list)
2. You have to choose two from Rio, Vidić, Stam & McGrath (maybe the latter wouldn't be a popular choice, but individually he was probably just as good as the rest — but, like with Robson, the team he was playing in didn't really match his abilities)
It's the "downside" of having a team that's been so incredibly successful over the years.
Regarding Edwards — aside from the ones who saw him live, I don't really like when people put him in. There's only one full game with him available (while you can get a very complete picture of the likes of Charlton, Best, Law etc.), and while it's pretty clear that he was already performing at the level of peak Roy Keane or Bryan Robson (probably closer to the latter in terms of his skillset), it's still more of a "what if" story. Although he was named 3rd best player in Europe (behind Alfredo Di Stefano and Billy Wright) at the mere age of 21... no doubt that he would've been a shoo-in in United's All-Time XI, and a very likely candidate for our best ever player.
As an outside thought, when you look at the 1957 Ballon d'Or list, it makes you wonder, how that 1958 World Cup could've gone. For those who don't remember, it was the breakthrough moment for a certain Pelé.
3. Duncan Edwards. 21 years old, died in Munich
7. Eduard Streltsov. 20 years old, imprisoned.
8. Tommy Taylor. 25 years old, died in Munich.
And all of them were in the same group as Brazil!