Buying and rehabing a duplex with fire damage

[Quincy Kea]

Hi Everyone, 

I'm fairly new to real estate and I wasn't sure where to start so I figured why not the forums!

I'm looking at purchasing a duplex for house hacking, however the one I'm looking at has fire damage on the bottom unit. From what I can tell the entire bottom needs to be redone. The top half has already been renovated and has a tenant in place. 

The bottom unit is about 900 square foot, I'm not really familiar with how to calculate what the rehab would cost. Do I contact the owner and have the them reach out to a contractor for an estimate? What are things to look for that would make buying it unreasonable?