If you just want to concentrate to play current Legion content, it is not difficult to catch up at all.

To play the game, you need to purchase both the base game and the current Legion expansion. If you purchase the Legion expansion online, you will be getting one free boost to level 100, which is the starting level to play current Legion expansion. That will allow you to start playing current expansion immediately.

And some tips. Blizzard should be having the mid year sales soon, and it is very possible they will give discount to WOW. The game sales should start in less than 2 weeks. The base game will probably be discounted either 50% or 75%. Even the Legion expansion might be offered at a discount. So if you wait a bit, you could save a bit of money.

Starter pack only allows you to play up to level 20, and the content is limited to the original base game content released 13 years ago. The total level in game now is 110 and there have been 6 additional expansions added to the game. Those low level content is really not a representation of how the game is like in the current expansion now. It lacks all the key features such as artifacts, class halls, followers, bodyguards, world quests, and end game raids. Not to mention most of the key character skills are not yet open up at level 20.

I suggest if you really want to see how a high level game is like, at least go buy the base game during the sales, which allow you to play up to level 100, and probably costs only USD$5.00. If you can find someone else here to level up with you, you can use the refer a friend feature to get leveling speed increases by 3 times for both of you, which allow you to reach level 100 much faster during the 1 month free play.

The expansion is likely to cost more though even with discount as the content is still being released incrementally. Even during sales it probably only has USD$10 discount so will probably still cost USD$40.00.

For me, the main reason to play the game is because this expansion is likely to conclude the story started back in Warcraft 3 RTS.

finally blizzard gonna offer 40% discount to Legion expansion starting now till 05/09/2017, now at USD$29.99. and next week a major content patch 7.3 will drop. if you haven't bought the Legion expansion, now is a good time to save some money.