Soil
Actually almost all plants would like this kind of soil. Unfortunately that is easier said than done.
How do you know what if your soil is "good light well drained' soil?
Really good soil is soft enough that you can easily push your spade or shovel down into the flower bed with about 2 to 3 hard pushes. If your spade bounces off the surface, you need to add lots of amendments before you plant. Humus, humus, humus! If you have real clay why not give up and plant your begonias in pots?