I don't think Moyes will lose his job but he and Wenger leaving would likely make things very difficult for us. Van Gaal is already rumoured to be United's choice if they sack Moyes and if Wenger leaves, whilst we might not be going for the same managers, we may find the club they get their new manager from are then in for the same targets as us. On the plus side, if Wenger left then PSG may get rid of Blanc, who might not inspire some of you guys but I think he's very good.
If we want to get Van Gaal it's vital we get it sorted before the World Cup, waiting until after the tournament only to miss out on him would be unbelievably stupid. Fortunately I think there's a strong chance Van Gaal's future will be sorted out quickly at the end of the season, whether he signs for us or another club.
Despite not having the PL experience Van Gaal is the safest choice that we could get imo. After that I guess it's more risky choices like De Boer, Pochettino, Laudrup, and possibly a Blanc or Low who could be interested in coming. Blanc's definitely the pick of the bunch but he's a long shot and if he does leave I wouldn't be surprised if he ends up at Barca or Dortmund(if Klopp gets the Germany job). I'd take any of them over 12 more months of Sherwood though.