I was a painting contractor for many years and never asked for a deposit upfront. A good contractor should have great credit with his suppliers and an even better contract if the deal should go south before the job is done. Get to know your customer and explain how each paint covers so they understand fully before you have to apply 3 or 4 coats. If you didn't bid your job right and the client has already signed the contract then that is on you not the client. Yes I haven had to eat the difference but the client was happy and a return customer for years so I made up for it.
I disagree with your criteria to hire a painter. A prompt returned call is nice but does not indicate the quality or fairness of the painter. As far as a written estimate, that should be more of a qualifier for the bid versus an evaluation criterion. I'm not sure one would have favorable results by hiring a painter on this basis. As far as the bidding process, change orders should ONLY be used if the customer requests additional scope (PMP 101). Angie's list should consider asking reviewers if/how much they were told to pay compared with the estimate. Unfortunately, there are a lot of contractors that are unethical and need to be accountable.
Thorough preparation is essential to ensure a good finish. Contractors might have to fill cracks or holes and remove old paint if it is badly worn, peeling or flaking. They remove all paint and apply a suitable primer to the bare surface. If the existing paint is in reasonable condition, they clean it and rub it with abrasive material to provide a key for the new paint. On new surfaces, such as bare wood or fresh plaster, they apply a suitable seal, followed by a primer coat.