you dont have to buy any cards to see their max stats if you go to the market click any card click any level that has a card for sale and click the listing it will change the default level 1 card shown into the level of the card clicked for sale.
while this isn't exactly as convenient for showing all levels of stats you can typically get the stats for 1st and last level of every card in the game using this method.
keep in mind most people havent started leveling up their cards yet so once they start doing that you will most likely find all level of all cards visible on the market to view in this manner