I just wanted to make a quick post to reassure some newcomers that Warframe is NOT pay 2 win, not in the slightest, based on my experience.
Every item in the game except cosmetics and more item slots, can be earn't through gameplay.
Sure, if someone spends a bit of cash, he may have the option to buy himself a different weapon sooner than you, but you can easily get the very same weapon with just a little time investment and no cash expenditure.
Seeing as how you don't fight versus each-other, it's co-op only, it doesn't matter who has what weapon, not until you get to really high level play, and at that point, you will have a sizeable armoury anyway.
I personally chose to give DE my money because what they had planned for the game really excited me, and it has provided me with a great many hours of entertainment and I felt they deserved the cash, but in all that time, I never felt like I needed to have spent anything, all I ever bought were cosmetics and inventory slots, none of which I needed, but I wanted just because I like having everything in the game.
In the interest of being transparent, yes, I bought a founders package (funded the original development,) have over 1000 hours in the game, am mastery rank 14, and am a Warlord in the Gaf PC clan.