Estimating ARV with limited comps

[Dan Ventura]

I'm trying to estimate the ARV of a duplex I'm looking to buy and having a hard time finding comps. I have looked as far back as the past year and there just are not a lot of duplexes in this market. How does everyone estimate their ARV when they are having a hard time finding similar properties to compare to? The house needs some serious updating to the 21st century but doesn't seem to need any major repairs. Currently 1 unit rented and 1 vacant with records of rent payment. Separate meters and separate HVAC units. Thanks in advance for any advice!