I suggest just buying the decks and familiarizing yourselves with the game before going deeper. Buying bulk lots you usually end up with a lot of garbage.
Once you understand the game a bit you’ll also have more of an idea of what you’ll need to tweak your decks.
Buying specific single cards you want is the most cost effective way to acquire what you want. It’s fun to buy a booster box or two to crack open packs, but that has a lottery aspect to it. If a bulk collection is cheap enough maybe, but like I stated above, it’ll be a lot of hot garbage.
To reiterate, buy some decks. Learn the the game. If what you have becomes stale buy more decks. Then once you’re familiar with the game branch out. It’s really difficult to explain what you’ll need/want to beginners, there’s just so much to the game.
Take my advice, and once you understand your decks come back with questions. You find that you’ll have more precise questions to ask.
Commander decks will probably provide the most variety easily. Assuming you want to play that format.
Duel decks are also good assuming you can find a good variety of them. Some of the older ones are better. The Duel Deck Anthology has fun decks, but it’s also pricey.
If you by any series of decks from the same time frame they will be “mostly” balanced.
All precon decks are subpar(IMHO). It’s best to by boosters from the same era as the decks to fluff them out. My play group we each got a different commander deck from Strixhaven, then we got singles from the same release to tune up the decks. They played out pretty evenly, but two of the decks showed more potential than the others.