"It doesn't have to be Nikon brand so long as it's a good lens. "

That's a contradiction. The only off-brand I would tolerate is a Zeiss...please, no holy wars here but you simply won't beat Nikkor glass (other than Zeiss) with any off-brand. And the prices have come down in the used market enough that it makes no sense from a money perspective to not get Nikon, either.

My experience matches most here, 24mm is about as wide as I've found useful - any thing more I find I crop in order to get what I originally envisioned.
That said, I'd think about the 28mm f/2 -- wide, fast and sharp. The 2.8 version is sharp also but why not live a little.