To help determine start up costs for your build out or remodeling you will need to know the square footage of the space. You will need to determine what it is you want done. Do you need to knock down walls to open up the space? Do you need to add rooms? What kind of rooms? An office? Restrooms (plumbing?)? Kitchen (plumbing, gas lines)? Painting? Wallpapering? Flooring? Ceilings? Lighting/electrical?

It will be dificult to determine any of this if it is still just hypothetical.

Also, your specific needs will be determined by whether the space is in "vanilla box" condition (new build, unfinished) or an existing or previously used space (finished).

If it is already finished but needs remodeling, you need to know what needs to be changed.

So, while the concept is hypothetical and you are looking for ballpark figures you can use very general numbers. Assume for example that you will need to carpet 5000 sq ft. Determine your costs for that (you can get basic low, mid and high ranges from any home improvement or carpet store).

As for needing a consultant...that could get costly. If you have that much money to work with then go for it. However, if you don't have your own money tree in your backyard then you may only need a carpenter and/or painter and electrician. You should ask friends and associates for recommendations. Depending upon your needs, you may not need a general contractor.

For vanilla box spaces, often the owner/landlord will finish the space to your specifications. Sometimes at your expense, sometimes at theirs. It's all figured in the negotiation of the lease. They may absorb the costs, but build it into the rental rate. They may take care of some of the build out and leave the flooring to you. Everything is negotiable.

Hope this helped.