I think you would be able to buy tickets day-of, but if it's important to you, I'd book ahead of time.
I took the tour in July and I was the only person on my tour! It was incredible to have a private tour, especially at the price point of the elephant tour! I would absolutely not count on that happening, however.
I agree with the prior poster's assessment of the distance. You're a bit closer than you would be on the safari, but you're not in a moving vehicle, which is beneficial for taking photographs that are not blurry.