Heating costs go up in winter, as does electricity due to the reduced natural light and more people indoors.

At some point they'll take over the business and do whatever they want with it (though 2 of the 3 already have the real estate "bug").

Second, look at converting to all electric. 

I'm also curious how much it costs to prepare the land and install plumbing, electricity, and concrete (driveway, walkways, patios).I'd love all the insights I can get from anyone that has experience doing this.

"wets" likes water, storm, sewer, and "dries/dry" like gas, electric, etc.

On top of that, you have other costs, such as permit, water, electricals, sewage system, etc... 

A 3rd floor addition will not only have structural modifications, but also require mechanical, electrical and plumbing permits.

Each one has exciting new projects on the horizon, like a minor-league baseball stadium in Spartanburg, South Carolina, entertainment districts in Huntsville, Alabama and Alpharetta, Georgia, and even economic development, like electric vehicle manufacturing in Bowling Green, Kentucky.FULL STORY

I also wouldnt worry about the electrical drain unless you are converting it to an MTR and paying electricity. 

Overhead door location and height, width and depth of space, bathroom locations and electrical and plumbing rough-ins etc.