Contractor during purchase?

[John Adams]

I have recently began my real estate hobby and my first deal (a rental) was a success, at least so far :). I want to complete a flip now. I have secured a preapproval for an investor's rehab loan. However, one requirement is that I submit the contractor's information and quotes along with the purchase agreement.

My question is (and forgive me if this is an ignorant one), how do I accomplish this during the purchase process? A realtor shows me a house and I have to have the contractor come with me? If that is the case, would I only get additional quotes after puchase is finalized and can I even change a contractor if on a purchase contract?

Thanks for your help!