http://www.singaporemath.com/Primary_Math_s/21.htm
I like the Standards edition.
Minimum IMO is text, WB, and Home Instructors Guide.
I also really liked Intensive Practice and Challenging Word Problems...although starting with Miquon and Cuisinaire rods may be best....the hands on in Miquon was really neat and the .cuisinaire rods go nicely towards bar models.