He's gonna want to step up to multiple nanoHD's or Pro's, or hell even the UAP - HD on each floor to get usable coverage. And no, do not go mesh in a business/corporate/enterprise environment.

I highly doubt, though, that there are going to be 500-600 wireless only users, right? Surely some of them are going to be on desktops that are hardwired into the network.

This isn't something you can half-ass and expect to get any kind of customer satisfaction out of. With this many users, you need to think big. The second you deploy those AC-Lites and that many users start connecting, they're going to catch on fire and melt. (Figuratively speaking, of course.)

Like my grandpa, and your grandpa and everybody's grandpa used to tell us when they would catch us using the wrong tool for working with wood or whatever "There is the right tool and the wrong tool for the job". You want to use the right tools.