For your situation, $40K for a building generator sounds about right. It should power only necessary equipment like servers, phones, lights, essential computers, and enough heat & air to keep a reasonable temperature. To power everything in a building like yours, you might be looking at $50K-$80K. I would suggest running on natural gas instead of diesel, no tank to leak or run out. You would still need battery-powered UPS's on the computers and phones to keep them going during the 2-10 seconds for the generator to kick in. For the server UPS's, get ones with external batteries so when a battery dies you can replace it without shutting down the server.
