Sure Contractors *can* do estimates/walkthroughs, but if they don't think its going to lead to business why would they? I've never had a contractor tell me the home has to be inserted first, but maybe that is just their way of seeing if your serious (i.e., if you're not willing to pay for a home inspection then they might assume you're not serious about the home, and therefore they may not invest their time doing a walk through with you).
Its definitely harder to get estimates/opinions from contractors without having a relationship with them first, and its hard to develop that relationship without doing business with them. I know its a chicken-and-egg problem, but its one that all new investors go through. Have you considered partnering with an experienced investor in Bel Air who already has relationships established?