I'll defer to someone else on the construction question... not my department!
I know from speaking with you (offline) that have been paying attention to supply / demand in your area. Initially, it sounds like you are in a great area!
Would you want to be the general contractor on your project, or hire that out? My suggestion would be to:
Run some basic proforma financial statements.
Income = What are self storage properties in your area getting per square foot in rent?
Expense = Insurance, Property tax, utilities, maintenance, advertising, etc.
Potential profit?
The next step is to talk to a few general contractors in your area. They would want to see some "plans" to give you a hard bid, but some of them may be willing to give you a "ballpark" number.