Yes, looks like you need to do some real due diligence to make an informed decision. For starters, you should get income statements for the last three years and the current rent roll. Insist on signed copies of tax returns. What is the economic occupancy vs the physical occupancy? Are there tenants who haven't paid in some time or are actual rents below posted rents? How do the rents compare with other self storage in the market? You should assess the likelihood of increasing rents and occupancy.
Analyze the operating costs. Do they pay themselves a management fee? Are there any personal expenses like car lease, cell phones, etc. included? Be mindful that real estate taxes are likely to increase as a result of a change of ownership. Will the facility need system improvements like online ordering and pay to make it more attractive? Can you add services like moving supplies, insurance, U-Haul?
Is there any deferred maintenance? How old is the property? Does it need a roof? Are the doors in good shape? What's the security like? Will you have to upgrade anything to improve the occupancy? What would be the cost? Does the property have any environmental issues?
Assuming NOI of 50%--it may be higher or lower for this particular property, the $650K ask is about a 7 cap. The investment group I work with who targets third tier markets looks for an 8-9 cap with reasonable prospects to value add up to a 10-11. However, this all depends on your local market. This is information a good buyer's agent should be able to give you. You can PM me if you want more info on how we look at properties.