I haven't actually played the
RPG (or the minis game either) but like you I like the setting and models, enough that I backed the
RPG kickstarter to get a bunch of PDFs for the fluff and artwork.
You don't actually need models for the
RPG. Combat can be done entirely 'theatre of the mind' style. It uses a version of Modiphius' own 2D20 rules system, which is used in several of their other games such as Conan, Mutant Chronicles and Star Trek.
The default assumption of the
RPG is actually that the party will be from different factions but operating as a sort of special ops team under the umbrella of O-12 - effectively the setting's UN authorities. Thus characters will likely have an overarching mission but might have their own secret agendas from their factions etc. You can certainly do mono-faction parties as well though.
The core book contains lots of information on the different factions etc, I would think easily enough to run a campaign with just that. If you want to go into more detail with particular factions there are sourcebooks as well, but you certainly don't need them to play.
There's an introductory adventure and quickstart rules available for free, so you can see the basics of the rules system:
https://www.modiphius.net/products/infinity-rpg-free-quickstart
Hope that helps.