One other thing that you might encounter is insufficient air for proper combustion when many gas burning appliances are located together; this can lead to backdrafting where air for combustion is pulled through the chimney (running the risk of exhaust gases getting back into the house's fresh air).

We didn't burst during the dot com bust. 

In the dot com bust, only San Francisco saw a decline.

Plan for a once in a lifetime event every 10 years... like a global pandemic, housing market crashing globally, the internet being invented (and then causing the dot com bust)... a group of terrorists slamming airplanes into two skyscrapers in NYC. you get the idea.  

It will require following some requirements such as class A roof, non combustible exterior finish, etc.

I typically use the terms intake and exhaust for air supply to a furnace (intake) and the result of the combustion being the exhaust that gets vented.

“Success is not the result of spontaneous combustion.

I second the approach mentioned by George Paiva above.If the existing boiler isn't too old, you might be able to convert that to gas by installing a gas gun from the likes ofMidco InternationalCarlin Combustion Technology Wayne Combustion Systems that way you would only need to add two boilers.

As long as the furnace is in its own room, with a fire door and outside combustion air vent, and return air drawn from main house, It should be good to go and doesnt have to have return air ducted throughout house.

There was an elbow outside and then single wall running up thru the eave with only two inches of clearance to combustibles.