Considering purchasing an occupied rental

[Michael Core]

I'm considering making an offer on a rental property being sold at about 80% of market value that is currently occupied at about $100 to $150 below market rent that that is under lease by a long term tenant until the first quarter of next year. The property has an unpermitted structure attached that is being used as a family room of sorts. The seller is a wholesaler looking to assign his contract and wants all cash. The property should cash flow at about 3% of the purchase price in the first year before income taxes but has major upside in selling after the current lease expires. My partner is concerned with the unpermitted structure and the 10 day close request. The area is a strong for rentals and retail sales. Other than an inspection and appraisal to validate the safety of the unpermitted structure and property value, are there any other suggestions with regard to ensuring the deal is worth pursuing? Thanks in advance for your insights.