What due diligence is necessary when taking over a property with existing tenants?
If you can get copies of the current leases, or the terms of the leases, that would be good. Sometimes, landlords don't like to turn them over. At least get the rent roll, when things are due, and if they are on leases or month to month.
Estoppel agreements if you can't view the leases. And sometimes even if you can. You need to know rent rolls, length of lease, is everybody paying on time, any "discounts" on lease (like lawn care, etc). Personally, if the seller isn't willing to show me the leases, then I'm moving on to another property. Sometimes they won't show you before an agreed upon price, but it could be written in as a contingency. If seller gets a "as is" offer, then somebody else can buy it.