Worth it for EU for sure, but the US battlechest+legion works out to be about a 4-5$ saving, which you'll find yourself charged that amount by g2a/kinguin for using a credit card.
edit:
It sounds like it will not work if you've already purchased a battlechest prior as it is also stated on g2a:
This product CAN be used ONLY on account without World of Warcraft game activated on it. (You may have enabled TRIAL version). Only one copy of Battlechest may be activated on each Battlenet account, please make sure that you haven't activated any on your account before buying this product.