Do you have to buy HoT ?
Dungeons won’t change with HoT. The only thing that probably will is the number of people running dungeons vs the number of people experiencing the new content.
Not really, no. You could still play a lot of the content without HoT, but I’d wager you probably won’t enjoy it as much when there are areas and aspects of the game you can’t access.
So in a way, if you’re investing a lot of time in the game, you basically will have to purchase HoT in the long run.
long run you might want to get HoT since a lot of your guild mates will be on it. Dungeons meh, they will see a sharp decrease in population for a while as veteran players are hungry to do something NEW! But it is your money so dont have regrets in your purchases enjoy them and haters gonna hate.