Rod building is fun when you have a goal in mind. Are you looking to customize the rod grip, build a rod with a specific balance, design your own guide spacing, or create a rod with an action that you just can't get off the rack? Go for it but take your time and do your research.
As others have pointed out, you'll probably spend more than buying a rod off the rack. But you get exactly what you want/ need. It's custom, after all..
Good luck,
Rick