You probably want to make sure it is up to code, ie. firewall , etc...check for permits pulled. 

You also want to put a firewall between your buy/sell risky projects and your portfolio projects held in the other entity.

Way I see an LLC now is an illusive firewall between me and a lawsuit, but that may change as I learn more.

From some other advice I have received, one of my biggest costs would be installing fire walls between the units.

The entrance needed to be separate, all plumbing, electrical, and heating would need to be separate and a firewall would need to be put in (mostly just an extra layer of drywall in the ceiling.  

It's not just the cost of putting in the lines, but the tap fee imposed by the water works for simply tapping into their lines, and other such expenses.Also, you will need architectural drawings before doing your pro forma, because things like firewalls can  make a huge difference in construction costs. 

On a couple of occasions we have encountered concrete block/brick firewalls between units.Dividing/Common walls are required to be fire rated - typically to 90 minutes.  

Most of the really good data is behind paid firewalls.

@Collin Chew I like to think of an LLC as a firewall between my assets and businesses.

Had fire walls and all and they were two story with a small balcony on the back overlooking the creek.