If you are looking for a contractor to build you space where you will conduct business it is very important that you select someone who has experience with these projects. Many times the building codes and zoning requirements are completely different that they would be for residential construction. Even a well meaning contractor could make some mistakes in your build out that would be very costly to you both.
I would advise you to engage either a contractor with Design/Build capacity or a local architect to help you develop a set of plans for what you want, and need to accomplish. You will need these plans for permitting and zoning anyway and in the end it might help you to get a cost effective design that meets both your needs and your budget.
Make sure you have a good written contract in place that allows you to be financial partners with your contractor so your outlays of money match up with the work completed.
As some other folks you trust like you accountant or your attorney to recommend someone or at least take a look at what you are doing too.
You need to get a NARI professional to take a look at the proposed building before moving forward. Please go to www.nariremodelers.com and locate the nearest professional in your area. If you want to learn more about NARI go to www.NARI.org and there is a detailed overview on how to select a contractor. The exact url is http://www.nari.org/pdf/How_To_Select_Brochure09.pdf and please print the brochure off as it will step you through the entire process!