You can get almost all the frames and weapons just from playing the game, and there is almost always something new available to you even if you’re farming for something specific. I wouldn’t recommend paying for weapons or frames.
What I would pay for is weapon slots and warframe slots. Those cost platinum, and you will likely want extra slots as you go.
I would also buy a couple of color palettes that you enjoy so you can customize your appearance.
But outside of that, I would just play the game and watch things unlock.
You can also get a free sword using the promo code: FREESWORD
It gives you the Heat Sword with a catalyst. The sword itself is pretty decent for the early game, then later you can sell it if you want and you get to keep the free weapon slot it comes with!
I’m sure that everyone here has their own groups, but I have a solo dojo i’ve been working on for anyone who is interested in joining and getting the research fruits of my labor. Clan is called ‘Bennonites’ on the PC and I’ve worked my through about 1/2 the research thus far.