Each of us two founders have at least +8 years pro experience coming from big well-respected brands/companies. What is the best to answer or to handle this question? I guess the possible client whats to make sure how we deliver what we promise, right? What to do/answer to fully get his trust on this topic?
Love the answers given so far. I'd add one more thing to your message to the brand.
"We offer a guarantee."
Make up a guarantee that will quell their fears, reassure them that they're not taking a big risk working with you. And remember, the guarantee does not have to be a 100% refund. There are lots of different types of guarantee - workmanship, timeliness, quality, expenses markups.
Go for it and enjoy the ride! I only started my agency 4 years ago and I'm loving it.