What league do you plan on playing in? That makes a big difference in which cards to get.

Check out this post which shows which cards are used most frequently by league and the win rates. Check your league and focus on some of those cards. Chaos Legion are the most inexpensive to get now. https://peakd.com/hive-13323/@jacekw/usage-win-rate-of-monsters-and-summoners-or-season-107-or-modern