If you want to buy a couple of booster boxes I'd suggest nothing prior to ultra prism. Ultra Prism onward is safe from the next rotation, we dunno if Crimson Invasion and Shining Legends are.
There is currently a pretty ok Charizard build going around, and you can get MOST of what you need for it from buying two of the Team Up Charizard theme decks (including 4x Cynthia, which is about as close to the old Professor Oak card as you will get in this format)
A lot has changed since you last played
Player who goes first cannot attack on turn 1, but does now draw a card (I dont remember the ruleset of base-rocket, but in the first EX era you couldn't draw on T1, but you could attack)
Trainers are now spilt up into four groups
Items (can play as many of these a turn as you wish)
Supporters (can play one per turn, unless an effect in play states otherwise - these replace the really powerful trainer cards like Bill, Lass and Prof Oak)
Stadiums (I actually think these MAY have been around when you last played, i am sure Gym series 1 had a stadium. These remain in play, you can play one per turn and they effect both players. You can use a Stadium that is in play in a turn and replace it with a new stadium, but only if said stadium was in play before your current turn started)
Tools (attach to a specific pokemon, stay in play unless stated otherwise or hit by effects of another card)
There are now different classes of Pokemon
GX - usually very powerful, but will cost you two prizes when they are knocked out.
Prism Star - Can only have one copy of each per deck, usually have a very good supportive effect (Ditto prism acts as a basic for any stage 1. You can also have prism stadiums, supporters and energy, which follow the same rule)
Regular pokemon (these work the same as the old ones and make up the bulk of cards)
Poke-Powers are replaced by abilities, which are a lot more present than Poke-Powers were
Other than that, the core mechanics are the same. Attach an energy per turn manually (though a lot of cards exist to speed this up), play any item or pokemon you can/want to, use any ability that needs to be activated and then either retreat or attack or pass turn.