Moyes hasn't been very creative in his signings, but I think he's been given a tough hand. Sunderland are saddled with debt (unsure if its external, interest bearing debt or debt to Ellis Short) and Ellis Short is now trying to sell the club, so they aren't going to splash the cash. Had this been 3-4 years ago, he'd been given plenty of money for transfers like previous Sunderland managers, most of whom have squandered a lot of the money.
Sunderland is a strange one. Everyone associated with the club describes a dysfunctional culture within the club that has almost come to expect struggling.