Monsterpocalypse has some of the better painted prepainted models on the market (bar Ubercorp models that are
painted ruined with a thick, enamel-like white paint). MonPoc minis are painted way better than any gashapon-like figure you can get on Ali Express. They're one of the few prepainted ranges that actually have shades, not just solid colors.
This said, I've collected a few of Kaiju-like figures from other ranges, for a similar project:
brown mecha from Robotech toy line, spare tripod from MonPoc, robot from HeroScape, rebased flaming dude from HeroClix, vine monster from WotC D&D, mecha-godzilla from MonPoc (this is a promo figure I snatched by luck, not easy to find now, I suppose), red and blue drakes from WotC D&D, greenish monster on four from WotC Star Wars.
This said, I have an extensive collection of MonPoc miniatures (inc Voltron) so I'm quite set for kaiju/giant robot needs =)