Rasoberry wheat is actually a common beer. I'd go with real berries instead of juice. I haven't done one yet, but plenty of folks on the forum have. You can probably find someone's recommendation by doing a search. Also the Brewmaster's Bible has recipies with as little as 3 lbs and as much as 6lbs.
If you do use juice, make sure you get 100% raspberry juice and its not-from-concentrate. A lot of juices are blended with grape juice, which will probably give you undesired flavors. If its from concentrate, it will have high fructose corn syrup mixed with it, which will also give you undesired flavors.
Planning: Agave Witbock, Raisin Beer
Primary: GF Hazelnut Stout
Tertiary: Cranberry-Pom pLambic (est. bottle date: 03/01/08)
Drinking: Cab.Sav/Merlot Wine, Grand Cru, Hazelnut Stout #3, Ordinary Bitter