I only use them left handed when I want to be able to arrow AND mouse at the same time...
You definitely will want to use the mouse. The control scheme kind of relies on it for precision building and aiming arrows.
If you just want to build, you might consider building a world in Creative mode. In Creative mode, you can't take damage, you don't get hungry, you can fly, and you have infinite building supplies. It's great for exploring Minecraft and learning the controls when you're still getting used to things.
Also, if you make sure to use the same "seed" for each world, you can make a pair of identical Creative and Survival worlds, allowing you to explore the world in Creative and then play it in Survival once you have a general idea of where you are and what you want to do with the world.
