Because of it's size, a CP is typically made out of MDF or Plywood. MDF holds paint better IMHO. Poplar would be my wood of choice (because of how it holds paint), but you would have to glue up panels, so very few people do this.
The width of your control panel will be determined by the space needed to accomodate the controls you choose. The width of your cabinet will be determined at least partially by the size of your monitor. If the width of the CP is wider than your cabinet, then you will probably build it separate from the cabinet and then attach it. If your CP will fit within the width of the cabinet, then my opinion is it looks nicer built into the cabinet.
These are my opinions.