Welcome aboard!
You didn't mention which card you wanted for signing up, so I bought you a Flamesmith from the Dice collection. (Seems like the most useful/powerful card below 50cents to me right now, considering that the Untamed cards are already playable as phantom cards when signing up)
You may consider quest potions, which can pay off but are a game of luck in the end, hoping for good loot chests from quests.
It is good to have gold and legendary potions when opening packs but you may hold off from buying them for now as they can also be earned from quests. Personally, I didn't open packs in months and my potions from quest rewards are overflowing:
Don't feel down from the small reward amount in Novice/Bronze league. When you rent cards and get to silver/gold, rewards will be much better! Keep in mind that you need summoners at a certain level to use levelled up monsters, so looking for summoners should be the priority.
Thanks! Didn't even think to ask for anything else, you've been really helpful :-)
I've already used Flamesmith quite a few times with success thank you so much! Managed to complete the daily challenge too so starting to get the hang of it. It's a bit addictive ;-)