I am able to build 2 units roughly around 800-900 sq. ft. which is about 1600-1800 sq. ft of livable space which I am able to find market comparables but obviously without the same lot square footage.

Pricing ranges wildly due to the different models so make sure comps are really comparable.

If the units are truly comparable, then you will absolutely want to base your ARV off the sale price of the updated units.

I have found that some brokers are hesitant about offering such a small number compared to a listed price.

The rent comparables, expense assumptions and capex budget look reasonable, although I must admit I have limited knowledge of the market this property is located in so I'm relying on online sources to verify rents. 

If you want a piece of mind, you can always pay for another inspection to compare.

See Chris' response -- you're living in one half, you need to live somewhere, so how does the amount you're paying compare to market rents? 

. $5 comparative market analysis - You give me the address, and I will pull up Active, pending, sold over the past year and leases within past 2 years.

In today's market, you usually don't make that profit on a fix-n-flip (for a comparable house).

Check this out and it breaks down the two options in more detail and compares them-https://www.biggerpockets.com/renewsblog/2015/04/1...