I ran some numbers for you and I would suggest against this investment. Taking into account your fully capacity NOI and comparing it to a 25-year return on the S&P 500 (10%) -- your cash-flows barely beat it out.
NPV of Storage Unit Project: $42,391.28 (Assuming full capacity and no hiccups / maintenance)
IRR: 11.15% (1.15% better than anticipated S&P 500 return)
Essentially, if everything goes perfectly -- you will have a $42,391.28 profit in todays value. Is that 42k worth the risk of being liable, dealing with really annoying customers, having to keep track of staff, etc? Obviously creating an LLC would be necessary to reduce liability, but that also makes you hop through more financial hoops (financial statements, different taxes, etc.). If you are not a financial person, this would probably have to be outsourced at a cost. My dad owned around 20 storage facilities and it is relatively profitable as long as no one else moves into your area with competition -- so that is a risk you must be willing to take.
Personally, I would much rather throw my money into the market and lose out a little without the effort of this storage facility. If you want a chance at bigger returns find some aggressive funds to invest in or just be content with a 1% difference. Good luck!