Only cards with the miscellaneous tag on TCGplayer qualify to be included among the most valuable here, with exclusive prereleases and one-off crossovers leading the pack of expensive Pokemon TCG cards that don't come from a set or subset
Green has some good ways to gain life but blacks sacrifice-for-life engines are the way to go
Second, as mentioned, there's a great tutorial system in place
The first piece of advice is to never break your word
Now, of course, there are many qualifying arguments here, especially if youre looking to build a deck and really need that fourth uncommon Horsea for your Water deck, or what have you
Coupled with the balance in the current metagame making more decks than ever a viable strategy for competitive glory, it gives players the remit to play just about anything provided they have the strategy and skill to upset and overcome an opponent