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If you're collecting just to have something to collect, I'd suggest looking elsewhere. Cards aren't really display friendly, they have little intrinsic value outside of the actual game, and as others have mentioned its a huge money sink for what boils down to pretty bits of paper. You need full force autism to really get any enjoyment out of just collecting them. Which I just so happen to have.
I simply adore the artwork, and organizing cards is relaxing for me, similar to how one might enjoy caring for a bonsai tree. But that is another big issue to consider, because once you get a meaty collection, actually putting them together can be very bothersome. Unless you don't mind just sticking them haphazardly in a binder, which if you don't then by all means.
I organize them numerically by set, but that's just me. And another thing to consider is the mind set that goes into it. Despite their limited actual collectors value, there is a sort of double edged sword in the fact you will never run out of different cards to collect and hunt for. So I don't recommend going into this with the mindset of actually "completing" any actual set. You're only going to drive yourself mad. Especially with all the Gx full body/secret rainbow vairents, as well as full body character artwork cards. it's nice to have a few of these, but only get the ones for Pokemon/characters you absolutely adore.Otherwise you're gonna double the amount of money you throw into this. You can still have an appealing collection without having every single card they print.
As for actually buying them, getting them as singles is the way to go. Outside of the for mentioned gx card, it's very rare for any single card to exceed past a dollar, unless it's competitively sought after. You're usually able to get more out of your money than gambling on packs. Plus it saves you the clutter of opening multiple duplicates. If you wanna go after packs, it should only be for the specific thrill of opening them.