We can comfortably unload Besic and Mirallas, provided there are buyers.
We can afford to get rid of McCarthy too but (1) who's buying a player who hasn't been fit for the better part of two years?, and (2) I have a feeling that Allardyce will want a closer look (pending a return to fitness).
W/r/t Klaassen -- I think there are things he does well (movement off-the-ball, one-touch passing, finishing) but his approach is a little too passive. We seem to function best as a team when our more progressive midfielders are eager to get on the ball and drive us forward. Klaassen doesn't appear to fit that mold. Allardyce and Walsh have to make the call as to whether he's a long-term fit. My guess is probably not.
I'd prefer not to let Sandro go unless he's dead set on going back to Spain. I think he'll have opportunities in the second half of the season and it's worth giving him a chance to show what he can do. As sort of an aside -- was anyone else irritated last night when Lookman decided to have a wild shot instead of passing to Sandro, who had pulled wide to find space in the box? The game was settled but I still found it annoying. We'll never know if Sandro is any good unless we start getting him the ball in the right areas.
I'd only sell Schneiderlin if a good offer comes in. I think he'd look much better as a lone DM, where his only jobs are to mop up in front of the CBs and lay the ball off to simply to his teammates (with the odd switch of play thrown in). Gana has earned that DM role for now but suspensions and injuries are hard to predict so there's no harm in having solid cover. With that said, if someone makes a serious offer we'd be silly to turn it down.
Who knows what's happening with Barkley? He's an intriguing fit for one of the CM roles in a 4-1-4-1 (which seems to be our best approach) but there's so much uncertainty around his situation. I don't think there's any harm in keeping him around until the end of the season and trying to convince him to sign, provided we think he's a fit. I wouldn't be surprised if the board prefers to sell, though.
I wouldn't consider selling anyone else unless a genuine offer comes in for one of the backup goalkeepers (which is very, very unlikely).