You basically can earn everything you need and want in Warframe with time and patience. I only bought a lilttle bit of plat at the start because I was collecting warframes and weapons and needed inventory space. You'll get a lot of the mods you need from just doing survival/defense runs and some of the basic store bought weapons you can buy with credits (in game money) such as Boltor and Kunai will last you for a good while until you get the stronger ones. Just join a high level guild so you can get access to all blueprints for the best weapons and you'll be fine.
If you play, I'd say get Mag as the starter frame, then try to get Rhino, Trinity and/or Frost.
