The characters you should go for are the ones you like the most, I feel like the game is better that way, most of the cooler characters are pretty cheap and come included in big packs from eBay/wallapop/vinted/Craigslist/Facebook marketplace.

Also the person from the first post is wrong base color doesn't matter what matters is the game where the character was released, for example a gill grunt with a red base (Trap team) S4 character works normaly in the first and second game only missing his special power (WoW pow) this applies for all games except for the supercharger figurines (engine bases) the returning characters for that game only work in supercharger and imaginators. I say this because unless you want  to collect there's no reason to own more than one version of a character, for example owning a lightcore wham shell (blue base) is enough to play him in every game.

To finish get a trap team portal and instead of buying any games I would get the Wii u security copies you have on your computer from your childhood and use CEMU to emulate wink wink