The lot is just inside the border of an area recently up-zoned to Residential Small Lot (no minimum lot size, 1 DU per 2k sqft).

While I’m in the real estate business as a residential agent, I feel a bit like a fish out of water with multi-family.The main question I have at this point is this: what documents should I be getting from the seller that pertain to the property during this DD period?

Once to get to five units, the lending goes from residential to commercial, and it's just not really possible to do a house hack.If you don't want to waste time, connect with a lender ASAP.

I am wanting to know about investing in residential plots.

I know of a specialty lender for Residential/Habitational properties.  

He works both commercial as well as residential sides and has many investor clients.

You could even FHA up to 4 units at 3.5% down. 20-25% down is mainly on 5+ units on the commercial side not the residential side.

His wife's father recently passed away and left 2 residential properties in NJ.

@Ryan Chatman 27.5 years is the number the irs picked out of thin air for how long residential real estate would last.

Also the political anti landlord atmosphere is now out of control.