I added drywall and turned an upstairs room (previously used as a sewing room) into a master suite by adding a walk-in closet.

On the surface, it sounds sketchy, but as others have said if the agent has the homeowner's permission then no fuss, no foul.Sometimes people just walk away and they don't care about the money. 

Does anybody have any idea or estimate, as to how much a three story walk up fire escape would be for a building in Brooklyn , New York ?

@Kalen Jordan, one difference between the two are the curbs and walks.

So--as you alluded to in your question--I would recommend you find a fire restoration company/contractor to walk the property with you and give you a good understanding as to what needs to be done.

The ones that are hard to find have the least amount of competition because everyone else is walking away.

You certainly don’t want to walk on wood with nails and such so if you do need to do the flooring earlier than typical, simply cover them with heavy weight red rosin paper to protect them.

I apologize for the long time it took to respond, I wanted to walk the property.

or simply just walk away? 

@Roee Hazut to answer question #1 in your local market you should walk the property to get a clear idea of what rehab is needed.