A goalkeeper might not be a priority this summer, but it will be sooner rather than later.
I don't think they'll buy a right back. O'Shea was doing well for them there before his leg exploded earlier in the season. Wes Brown can fill in there okay, and Rafael is only 19 - I'd be surprised if he wasn't given more time. If they buy a defender, I'd expect it to be a centre half, and to coincide with either Vidic or Ferdinand leaving. Ferdinand's gotten injury prone, and there have been rumours about Vidic having a bit of wanderlust forever (I feel like he's had a lot of injuries in the past couple of years too).
Up front, they've Rooney (probably their best player now), Berbatov and Owen, and rarely play more than one of them. They've bought Hernandez, too, and there are younger guys there - Obertan and Macheda. I don't see another striker coming in.
The biggest potential for buys is in midfield. Scholes and Giggs are getting long in the tooth and Hargreaves is a crock. No one seems entirely convinced by most of their other midfielders, but I think Ferguson will stick with them, and just add one or two over the next two years.