Can I buy my ticket to go to the top there or do I need to buy it in advance? How long does it take to get up there?

You can buy it there. The elevator ride is approx 45 seconds. Depending on when you go it can get busy. We were there a few days ago and from ticket purchase to the top the whole process took less than 10 minutes.

You can buy it here but you get an allotted the slot, o can be on 15 mins or an hour from when you buy. And it takes about 15 minutes to get up there.