I am 23 living in Dayton and I am a Designer / Draftsman with an architecture degree. 

I am comfortable with managing the rentals, collecting, evicting, repairs, etc... 

If your lender is comfortable Giving you the money Before you have title, no problem.

. - What about designing a lease option for 12-24 months?

My question is whether anyone with experience of adding similar small interior storage units/lockers can offer any recommendations regarding design/materials/systems?

As in the design work. 

I started on my real estate journey right out of Architecture school, ultimately to get to designing and developing my own properties from the ground up.

I know that you're going to be able to get the construction financing and the cost of money and length of outlay probably won't mean much even if you're paying 12-14% as @Jay Hinrichs pointed out.Plus it's above most lenders comfortability on land even if you have the takeout. 

@Ned J.Retirement accounts (self-directed or not) are designed to provide you benefits at retirement, they are not for your current benefits.

FHA loans are for homeowners, and a LLC is a business, so there's a bit of a conflict if a business is trying to take advantage of a programs designed for homeowners.