The best bet, for now, is a box of boosters, and you can pre-order a box for the latest expansion, Vermilion Bloodline, for around $90, bringing you right up to date, and likely netting you some of the rares thatll attract big bucks in the near future.
Knowing when to use your disruptions is your greatest asset
From what I gather (besides that the youtuber mentioned does not endorse this practice) is that the Pokemon company did two things with the Sun & Moon series packs
This is especially true in decks that have other uses for creatures outside of attacking, like sacrificing them or tapping them for mana with abilities like convoke
Thats exactly what Purity does
All three grading services now distinguish notable variants on labels (Red Cheeks vs Yellow Cheeks, Thick Stamp vs Thin Stamp, No Symbol Jungle holos), so the slab itself confirms the pedigree