If not spending a ton of money is a priority, then singles are most definitely the way to go. Honestly, sets these days are huge and the likelihood of you pulling whatever you needed/wanted is fairly slim. That being said, cracking packs is fun. If you wanted to buy sealed product Sun and Moon and Sun and Moon Guardians Rising are the most recent sets and both have quite a few playable cards.
Preconstructed decks are really only going to be helpful for Trainers, and they won't be that helpful to be honest. Definitely better off buying staple trainers from someone/somewhere.
There's a Tapu Koko box out there with a promo Tapu Koko that is seeing a lot of play. Tapu Bulu GX is only available in a tin and it's fairly playable as well. If you can find them, Volcanion EX came in a tin and that's always been a pretty good card.
My advice overall would be to snoop around online for some decklists (we just had a bunch of regional tournaments so there should be plenty floating around,) pick a deck or two you like, and buy the singles to build them. However, it is worth keeping in mind that we are nearing the end of the competitive year and a few sets will rotate out of the standard format soon. We can't say for certain which sets those will be but it will likely be at least Primal Clash (along with Double Crisis,) Roaring Skies, and Ancient Origins.
