First of all, I'll offer my evaluation of each of the products you mentioned. This evaluation comes from many years buying Magic (since 2010), during which time I made many purchase mistakes I learned the hard way. I also follow content creators like the Professor of Tolarian Community College who offers good insight on products as well.
Deck Builder's Toolkit — These are great for a first purchase, as they give you a selection of cards that'll be usable together, along with some guidance on how to use them. The land packs are also nice to ensure you have enough lands for decks you build. But ultimately, they're basically $16 for the 4 booster packs, plus $4 for all the extra cards. Not that interesting once you have a solid collection going.
Bundles — These are pretty much $40 for 10 booster packs, which is the exact price of 10 booster packs off the shelf. Sure, there's some "fun extras", and if you like them, awesome. Like with the Toolkits, the land packs are nice if you're just starting out. But pretty soon you'll own more than enough lands, at which point there's usually not a great reason to buy these. That said, they make for great gifts!
Prerelease Kits — If you can get these during prerelease when the price on them isn't too high, these are a decent purchase. They cost the same as the 6 boosters they contain, but they also have that date-stamped rare or mythic. But see "Booster packs" below.
Challenger Decks — These are very solid products for their intended audience. It took a while for Wizards to get this right, but these are solid standard decks that are semi-competitive out-of-the-box. But that's the key.... They're for competitive play. If you have no interest in the competitive scene, then you could probably get the same game-play experience for cheaper. Also consider that their value plummets once the cards inside of them rotate out of standard.
Brawl — I have not tried these, so I can't say. But I've heard they're fun. But certainly not the cheapest of options!
Planeswalker Decks — No....... oK, fine, they're good for beginners, and in fact are specifically designed to be the entry product, with Deck Builder's Toolkits following them. But you can buy waaaaay better casual decks for the same amount of money! Don't buy these.
Booster packs — This depends on your goals:
Do you want to focus on buying / making fun casual decks to play games with? Don't buy boosters, just buy the individual cards.
Do you want to collect all the cards from the set? You can get a good part of the way there with boosters. Two booster boxes usually gets you playsets of all the commons and many of the uncommons. Then you just have to buy rares / mythics individually to fill in the gaps. But buying common and uncommon sets, as you did, is a much more cost-effective way to get those rarities, and the money you save doing that instead of buying 2x booster boxes will likely get you most of the rares / mythics you may have gotten in the booster boxes.
Do you want to use the boosters in a draft with friends? THIS is what boosters are for, so then yes, buy them! BUT.... don't buy individual boosters. It is much more cost-effective to buy an entire booster box. To get 36 packs off the shelf would cost you 36 x $4 = $144, whereas you can buy booster boxes for $90 - $100 most places. That's more like $2.75 per pack!
So in summation:
DON'T buy Deck Builder's Toolkits or Planeswalker Decks.
MAYBE buy Bundles or Prerelease kits if you like the promos and extras, but otherwise just buy booster boxes.
ONLY buy booster packs if you are drafting them. Otherwise, buying c/u sets or individual cards is cheaper.
IDK about precons like Challenger and Brawl decks. That's up to you ultimately. They are both fine products for what they're trying to do.
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Second, here's some ideas on affordable deck options.
Wizards sadly discontinued the popular Duel Decks product line, but you can still find several of them for close to the original $20. They contain two 60 card decks which are designed to play against each other, so $10 per deck — not too bad! Here's a list on TCG Player. If you sort the results by Price: Low to High, you'll find that there are about 4 you can still buy for $20, and about 3-4 more you can buy for $25.
Another really great option is Card Kingdom's battle decks, which are custom-created to be balanced against each other. They're also $10 per deck. Card Kingdom no longer displays the decklists on the product pages (because tweaking the decklists, as they like to do when new cards are released, made keeping the product pages up-to-date more time-consuming for their team). However, I believe if you message them, they'll give you the current decklist of any deck you're interested in, and often you can build them yourself for a little cheaper (especially if you already own some of the cards!).
If you'd rather build your own decks instead of relying on premade decks, then the Pauper format might be of interest to you. It has a lot of the power and complexity of old Magic formats like Legacy, but since it only allows commons, it is by far the most affordable format. Even the best, top-tier competitive decks only cost about $50-$80, but you can absolutely find strong and interesting decks for cheap.
Finally, a great site for looking up decks or building your own is TappedOut, where you can search for specifically budget decks. The site also offers a nice playtest feature, so you can play a few test games against yourself to decide if you like the deck or not. Then if you like it.... just buy all the cards you need! You can easily find decks for under $10 there.
I hope all these tips are helpful, and I hope you, your girlfriend, and any friends you convince to start playing have a ton of fun! Let me know if you have any further questions. 
Last edited by meldon44 (2020-03-09 19:59:14)
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