just from my experience, if you mean it seriously with HS, buy rather more than less. You will save your money. (But I really spent a lot this year)
What I could recommend as well is to purchase packs not for your currency (not via PC), but via Android - where you can pay by Google Pay.
On Google pay, you can make a profile, where you can link your card with another currency. I use Mexican Dollar MX$, which costs me 30% less than purchasing in EUR, so my address in Mexico is virtual (you can do it here) and normally link it to your current card on google pay profile. When I want to purchase anything, I close HS on PC, start it on Android, where is my Google profile linked, start the Shopping inside Hearthstone - shop icon, and voila - you see the prices in MX$. (Double check how beneficial it is before purchasing). 30% savings is not the bad deal. And it was verified with Blizzard Support, if there's no issue with it, if I am resident of EU and pay in MX$ - there is NO. It's all OK and legal. I am emphasizing once again - it's purchasing inside of HS game, not via Shop on web, where it's not possible. It means, it won't work e.g. for WOW.
Once purchased, you can turn the HS off on your android and start again on PC, enjoying packs opening. If you want to go this way and struggling, I can send the step-by-step video.
What is also important to know is on first 10 packs purchased from any expansion, there is legendary guarantee.
Attaching the picture how much it costs in MX$ for you to compare.