How to do due diligence on occupied 17 unit property?

How to do due diligence on occupied 17 unit property?

I'm looking at a 17 unit apartment and would like to know who I can hire to verify the numbers are what the seller say they are, as well as obtaining a letter of estopple from each tenant? Or can someone do this and not get compensated until after the deal goes through?

Thanks!

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