And as far as the work he's already done(drywall/floors), since there is no recent permits for plumbing or electrical., is it safe to assume that work will have to be taken out to be inspected?

Reconfigure 2 non-permitted rooms into a large master bedroom.

The previous owner added 2 additional un-permitted units in the basement making a total of seven units.

I'm fairly certain it was just built this way, so I'm going to search for old permits, blueprints, and anything I can find to learn more.  

We’ve just completed site plans and permitting for a brand new Self-Storage development here in Austin, TX.

The Town 'dump&pump' inspector showed up, apparently triggered by the sale of any homes with septics, then the fun & permits started.When they had the 20 year old unit inspected & pumped by the local septic guy everything passed.

For leases entered into on or after July 1 2020, the owner would only be permitted to occupy the unit, if the renter agrees in writing to the lease termination or the lease includes a provision providing for lease termination based on owner or owner-relative occupancy;Withdrawal of the rental property from the rental market;Intent to demolish or substantially remodel the unit;Owner is complying with a local ordinance, court order, or other government entity resulting in the need to vacate the property.'

You may have a variance that allows multifamily, you should be able to see it on Atlas if so, under Licenses and Inspections > Zoning Permit Documents.

How long on a permit?

Another problem would be paper work, permits contracts etc.