I'm not as familiar with Ft Worth but here in BCS my fourplexes only cost about $50-75 per month for water.

A roof on a typical duplex may cost $8k to tear off and replace.  

In your case, this appears to be $800k - $600k = $200k.You made no mention of the rehab costs.

The wear and tear on hardwood floor - carpets has no recourse beyond the deposit damage if there is pee or replacement costs needed.

The buyer can add a performance deed to the transaction which basically states that if the buyer defaults on the loan, the property is transferred back to the seller without a long and costly foreclosure process. 

Remember, that it costs money to refi...if its a small amount of juice to squeeze, I would just run your numbers to take a look at what makes sense for you and your journey. 

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So they'd loan about 450k if your ARV is 1M (assuming this ARV is accurate), and then you can get a gap lender for the difference (180k plus closing costs/points/etc, holding costs).  

What is the average cost for purchase, installation, and the associated app?

Is there anyone that would be willing to look at this property and tell me if it is worth the price and also to give me a rough estimate on what to expect the costs to be?