i don't see where di maria fits into a 3-5-2, van gaal needs to play a 4-3-3 to get the best out of him.
personally in a 4-3-3 i'd play him in midfield. but with utd's current midfield options you probably can't. di maria might be able to make it as a wingback due to his work ethic, but i have no idea whether his defensive contributions would be up to it.
i don't think utd's particularly need him, or that he should be their most important in terms of fitting him where needed, but it seems to be a signing of financial muscle. a 'we're still here' type of signing. if he's the last signing utd make this summer i don't think anyone in the top 4 will be too worried. still many holes in that squad, and di maria can't possibly fill them all.