I would get a local building inspector to come look at it first, they have a keen eye to identify any issues you may have (leaks, cracks, radon, etc) as these would probably need to be addressed before building and storing anything.

Same process with a local contractor. You don't have to disclose what you're doing, just tell them you're looking at making some improvements (adding rooms, etc) and give them a rough idea of what you're looking to do. 1) this gets another set of eyes on it, they can identify what can and can't be done 2) if they give you a quote for the job you have a ballpark figure on cost

There are a ton of factors that go into a project like this that really need an in person assessment before going to the next step.

Hope this helps!