I think classic is the best investment. Once you know what deck or decks you want to build, you can then narrow down whats the best set to buy from. Classic still has a lot of strong cards used in a wide variety of decks.
Okay cool. I'll go with classic. Didn't get anything good :-(. Then I got the 100 from doing the expert practice and got a legendary but for warrior :-(. Luck is not with me.
Anyways, played a bit online and up to five wins now with my daily quests done or nearly done. I'm seeing that a lot of people are playing the mage online which makes me what to play another class...