[First Purchases/questions?]

Been playing for a few hours now and I'm not sure what to buy first...

I went with Loki as my first warframe for some reason and it is working fairly well until I get to Infestation...

Then I run in to ammo shortages. So I was thinking about getting the Bronco or Lex because it looks very ammo efficient. I'm leaning towards the Bronco because it is a shotgun and I also wanted to get the Paris so it would be a good balance between the two.

I was also wondering what I should get with the 50 platinum I have?

Also, those inventory items on the market that boost affinity and credits, are they permanent purchases?

Is the web browser account linked to my in-game account or are the in-game servers just being dodgy because of Stema? When ever I'm logged in online, I get booted out of the client. -_-

Thanks in advance for your help!