@Gerald Turner hopefully you got an estoppel certificate outlining the rents, security deposits and any verbal agreements between the landlord and tenants.

Make sure their license is valid (check with your state’s MHIC licensing office) and get a copy of their liability insurance certificate (make that a requirement for them to get their deposit) before they start the job. 

Which one of us needs to seek a certificate of occupancy?

If it doesn't have a lead certificate on file, it IS going to cost you to delead, the state has pretty much made it mandatory at this point, at least if you want to avoid the liability of a lead paint lawsuit from a past tenant.

Especially ask about cost of insurance and whether getting an elevation certificate is worth it.Here's the FEMA portal to determine flood zones: https://msc.fema.gov/portal/ho...If you have a 3 bed rental with at least 1.5 baths and no water view, you should be looking to get at least $2,000 per month and if it's suitable for weekly rentals in the summer, roughly the same amount per week. 

If you converted your SFH to a duplex, were there any unexpected problems/surprises on the legal/regulatory side, like being unable to rezone the property or get a rental certificate? 

They sign off on the occupancy certificate which allows you to officially live in it. 

If the seller is providing a clear Certificate of Occupancy then the property is turn key and not in need of major rehab.

That being said, I would make sure you do not need a rental certificate to rent out the basement.

I am in the midst of getting the certificate of zoning but as it currently stands zoning is for a 2 unit and I want to get the certificate for a 3 unit before I close.