If you will have a human door that opens and closes like a regular one, whether you buy it or build it, I would not worry about the chickens flying out when you go in. That has never been an issue with my coop, but pay attention when you enter or leave. If you want that area for storage go ahead. But I would not go to the extra expense or work just because I was worried that the chickens might escape.
I agree.
Once in a while a bird -- especially a youngster -- slips past me when I'm bringing buckets in and out, but with 2 people available I can always recapture it.