How do you calculate total drag on a wing?
Typical values of e are between 0.7 and 0.98 in subsonic flow, and 0.3 to 0.5 in supersonic flow. Since it is in the denominator, e cannot become 0 or drag would be infinite. A square wing has an e anywhere between 1 (with aspect ratio close to zero) and 0.8 (with high aspect ratio and away from the design point for the twist distribution)

As a rough first-order assumption, the zero-lift drag coefficient is constant over the moderate AoA range when the flow is attached over the full chord. This drag is the sum of friction (shear forces tangential to the wing surface) and pressure drag (caused by pressure on the backwards-facing sections being lower than on the forward-facing sections of the airfoil, again due to viscosity).

Of course, once you have a full airfoil polar, your results will be more precise if you use the particular value of the drag coefficient at the specified angle of attack. However, sometimes you will need to calculate lift and drag without knowing the angle of attack, and then you fall back to the first-order approximation. For an example where this assumption is useful please read this answer.