I would advise trying to find information about specific family members, where they lived, and their places of birth and death, then seeking out these places to visit on your trip.
Speaking in general to someone from a genealogy society or similar may provide you with only vague details or nothing about your family members.
If you do your research before you go to Scotland, you will know more about the people and places and probably have a more rewarding and fruitful experience.
May I ask if you already have much information about your Johnston and Laing families or is the information you have given here all that you know about them? I presume you are aware of the site Scotland's People? Have you searched for parish registers relating to these people?
Keep in mind though that at the dates your Agnes left Scotland there may not be much to find about her and her family.
Added: I have just seen your earlier posts about this family.
As you are unsure of their place of origin and you are dealing with common Scottish names, unless you can find some clues at your end, you might not be able to go any further with this line. Have you thoroughly searched all the available records in the USA? Obituaries? Death/burial records? Newspapers of the time? Births of children which might say where the parents were born? Immigration records which may say where they hail from? Anything else?
