Just play for fun

You don't really need to spend money, but the Pokémon aren't that great either. For example, Charizard's ultimate is a single-target attack. If it doesn't defeat the opponent, it can continue attacking, but if it doesn't, it's just that one hit. It's good early on but average later. Lucario's normal attack hits multiple times on a single target, and its ultimate is an all-target attack. It's pretty good but hard to land. Some S-ranks can evolve, some can't. Keep all S+ ranks. For S ranks, keep the ones that can evolve or ones you really want, and use the rest as fodder.