I am 99.9% sure she can't go to Downingtown. You need an address that is in the district or she can't attend. I DO know people who used a grandparents address to attends Dtown (i went there) but if you get caught, that menas trouble. We just bought a house and sucked it up and paid a bit more to be in Dtown than Coatesville, even though you can find much better homes for the same price in Coatesville district. I suggest looking at homes in Downingtown WEST schools....tend to be a bit cheaper than Dtown East are.
As for a charter or private school....if you love the house, these are 2 great options and are easy to research once your child is around 4 yrs old. Just remember, EVERYONE on your street/in the neighborhood will be at Coatesville, and you'll probably have a hard time convincing a 5 yr old that she wants to go to a school where she won't know anyone. I heard the elementary schools are pretty good, it's just the middle and high schools in Coatesville, so you could try to transfer her then. but again, what 12 yr old wants to leave her friends?!?!
good luck in your decision!