Well, it takes like 10-15 minutes to level a fresh char on t6 to level 70. Thats easy.
And yes, I would buy the expansion.
The reasons were mentioned above. I play every season (more or less, depends) and its really fun. I played like 300 hours this season because it makes a lot of fun.
Compared to the last season: like 20 hours. But as I said, it depends on the situation and if I play alone or with friends etc.
After a lot of hours I try different chars (DH is main atm), e.g. as a main a DD and afterwards a Supmonk or another DD for different types of builds as Rebel mentioned.