Hi Jason. The most important numbers you should determine are the property's NOI (net operating income) and a market cap rate for your area. These will both determine how much you should offer for the property.
NOI is calculated as annual income less annual expenses. You figure out the annual income by determining the total potential market rent for the units by doing research on what similar units are renting for in your area. Don't just trust what the current owner is telling you, you could be missing hundreds or thousand of dollars of value. To figure out annual expenses, you should request actual financials from the owner to see how much they currently spend to operate the property (repairs, management, utilities, marketing, etc). But again, don't just trust what the owner sends you. Do your own research on what it would cost to efficiently operate the building. You could reach out to local property managers, brokers, or investors to find out typical costs. For a quick analysis you could use a 35% - 50% expense ratio.
Once you have your NOI, you divide that number by a market cap rate (typically 6% - 10% depending on your market) to determine what your purchase price of the property would be. You can figure out a cap rate by doing research for comps online, or asking a local broker or investor.
Side note: if you are considering building a shed or storage units, you should research the property's zoning laws to determine if that is allowed.
Cheers!