Mobile Security Tower Business

I’m looking into purchasing or starting up a business renting mobile security towers. I’m interested in feedback regarding the opportunities and challenges with this type of business. Specifically, how long are these contracts? Is there a standard third party to outsource the surveillance and response? Is the opportunity in selling the towers or leasing?

I don’t see many of these businesses for sale, so I’m wondering if that demonstrates a solid niche or lack of overall viability.
Anything else that would be relevant for this industry that I’m missing?