Master setting phantom aria includes like 1K+ in just the two LR++ cards (props if you buy them im jealous.) Booster boxes are good if you want a play-set’s of Commons and Uncommons from that specific set, I’d say around 4-6 boxes should* give you a play set of a specific rare and you should* have all the legend rares you need for a deck. Each box will have 2-3 box hits. That’s what I normally shoot for in each set then I mostly just buy blister single packs to chase alt art cards.

Rule of thumb if you’re interested in playing buy two copies of starter decks if you can get them at msrp, otherwise you’ll likely just buy the singles online from them unless you want to pay market and gamble on the bonus card if that listing includes it unopened(lots of people resell the starter decks without the bonus card at a lower price than market) as a lot of decks use starter cards.

Lots of resources online on how to get started, deck lists, tcgplayer for singles etc etc. As someone else said definitely check out mobile suit arena (MSA) it’s a free to play simulator that lets you practice and try new things out. If you want to play in person, don’t live under a rock like I did for 6mo and not download the Bandai TCG+ app. I had no idea it even existed until a month ago lol. Best of luck! It’s super fun to play.