@Russell W. Thanks Russell for detailed response. Yes, by boiler I meant hot water heater. So one hot water heater and one furnace for the 6 units. Electric for each unit is separate with 8 separate meters. Few additional questions related to this:
1) Does one run into issues related to capacity of hot water by having a common hot water heater- where one resident uses more hot water than other and another resident running out of hot water? Not sure if this is something you have experienced in your multi family buildings?
2) What is the protocol for maintenance for these items - maintenance done twice a year, once a year or one in two years? Online research recommends performing maintenance atleast twice a year for a hot water heater. Not sure what maintenance expectation i should be looking for these older houses.
3) Good to get your opinion on galvanized plumbing. Wouldn't it be an extremely expensive affair to replace entire building's plumbing with copper and/or PEX piping? If one finds out the building had galvanized plumbing - would you negotiate the price to factor in the expense related to replacing copper/PEX piping?
4) Also, i am not sure if the building has steam hot water heaters - I will find out more and keep in mind your point regarding the cast iron pipes.
5) Excellent point on mortar joints. The building i am looking at is a brick building. Who does the inspection for Mortar joints - is it the general inspector or does one need to hire a professional for this?
Thanks once again for detailed information. Very very helpful.