Product-market fit will solve your IT problems. Twitter had laughable infrastructure relative to its growth -- the butt of daily jokes, and it didn't matter at all. They had money to hire the right people and make the changes they needed to.

My advice -- keep it simple. Read http://highscalability.com for stories on how others did it -- see YouTube for lessons on simplicity.

Thanks for feedback! I'm more concerned about employee collaboration (office365 vs google apps vs ???) and hardware (mac vs pc vs chromebook vs ???) at this juncture.

You're worrying about the wrong things . . .

You'll need to choose all those based on what fits you, your CTO, your dev stack, your team and your culture.

Just throwing out some of my preferences . . .

mac, digital ocean, grasshopper, trello, google apps . . .