Above, there was what might be the key to this.
What you probably want to do is to have a Secure Storage Facility, and offer different levels of security.
Part of that is Zoning. Municipalities that have zoning, will have a zoning classification of r secure storage (it will be presumed to be climate controlled). Those which only use IBC or some form of IBC, will create some bottlenecks in what form of Class S Occupancy. (Note, you will have to have language that bars storage of ammunition and powder--or you will wind up in H-5 or H-2 Occupancy, which is a lot more expensive to build--and even harder to lease and Tenant Improve.)
In all likely hood your best bet will be Type I construction--I'd start from a premise of all CMU construction with either precast floors or concrete plank construction. I'd figure on having a range of cubicles, with 4", 6", and 8" thick partitions to increase the security of each progressively. I'd probably start with a 60/30/10 split on those. The thin partition walls would be most numerous and least "protective" (not that smacking down a 4x16x8 partition with #3 bars vertically at 16" O.C. would be all that easy).
I'd layer the security. You'd have to have a gate code or card. Then a code or card for the entry door. I'd probably have a card/code protected door at each of the levels of security, too. The customer would then be on the hook for how to lock their individual unit. You would have no master key--only the ability to lock the customer out for non-payment. That gets you out from under any handling of customer items--a key element of keeping your liability costs in control. So, that would look like a door with a deadbolt (your use) and an empty hole for the customer's lockset (their use).
I would not buy a single safe, let that be a customer choice. Instead, use the best most secure doors with frames anchored and fully grouted into the walls. Springing for "vault style" doors on the most secure units might be a nice touch to help those upsell.
Which going to wind up being an expensive building to build, no way around it. Even with almost no interior finish, this is still going to be $175-210/sf to build. Which means your architect needs to look into getting the modules precast, and merely stacking them. Which would leave only yhe corridors, stair towers, and elevator shaft to build on site.
The real question winds up being how much will it cost to climate control the building. Also how much will it cost to hire a reliable person to monitor the building, the parking lots, and such during the business hours. You will already be on the hook for a spendy fire alarm system (the fire marshal is not letting you build without one); you may be on the hook for a security system and a heft license fee to the local PD for that, too. You will also have to keep up with all your security equipment, too.
As a customer base, I'd advertise heavily among the professional set. Architects, engineers, lawyers and the like have all sorts of need for off-site storage of documents (and, increasingly, electronic storage devices). Those people may also need to store personal items of value. That cohort will also know people who have the money to pay for all this.
That's my 2¢ at least.
