Model for ADDING a furnace and water heater

[Pierce Conway]

Greetings All! 

I am a realtor in Northern NJ and am helping someone scout out this property in Glen Ridge. It is an older 3 family home with only one furnace and water heater. 

Im sure it is easy to create, but does anyone have a a good stock model/spreadsheet to underwrite the costs of adding a furnace and water heater? 

ALSO - anyone have any great success, or horror stories in their attempts to adding mechanics and shift the payment responsibility to the tenants? 

Thanks in advance!!