We've collected all that data on the size and price of the other duplexes on the street, which is good to keep records of. 

If you don't have a lot of capital to do it on your own, I would team up with an experienced syndicator - he / she has the track record and helps raises the money and you hustle to find the deals.

The only closing costs you will have will be those to close the transaction without any of the mortgage recording, down payment or underwriting costs.

In reverse order:3 - Clouds on title - Moot issue for a properly recorded instrument since the issue will have superior lien status.  

I'm sure there are reputable ones with track records and such in that area, but if this one is new to you I'd be careful taking ARV advice from them.

If there is no mortgage/trust deed recorded in connection with/right after the deed, it was a cash purchase.

-Search county land records for properties about to be up for refinance.  

@Patrick Jacques there is no clear record, but the house was likely built in the 1800's based on the methods.

Have him record a Trustee's Deed with the County.  

Is the CFD recorded and what type of deed will the OP get upon purchase?