First, if you are not willing to be the "general contractor" yourself, meaning, talking to and hiring specialty workers, making sure the timing is correct, etc., then hire a General Contractor. Some people are good at just hiring our specific tasks, but I find it easier to hire someone to come in and do the whole project.
Generally, I wait until I have a property under contract to bring a contractor out to walk the property and give me quotes for the work I want done. Occasionally I will ask a contractor before I get a property under contract for a rough estimate on certain things if I am just not sure what it would cost, but I usually send along a video walkthrough or pictures. Some will give you the numbers, some will want to look at the property and may or may not charge a fee for the service.
I will say, if you start bringing more business to a contractor and develop the relationship, they are more likely to help you out and walk the property with you. However, by that point you probably start to have a good idea of the rough costs and can put in an offer using rough numbers before needing to get quotes from a contractor.