Yes and no.
All the high N7 players are good. I've played with most of the players in the top 10, they're all good.
Most low N7 players are bad. I die a little inside each time I see an N7 below 1000.
You don't need a high N7 to be able to play the game, but having an ultra high N7 ain't bad. In my case, I can say that I'm the best player in the world, simply because I have the highest N7 rank.
Credits come from playing thousands upon thousands of platinum speed runs. We used to be able to complete platinum games in 9 to 12 minutes. A a result, we were able to accumulate hundreds of millions of credits and spent a sizeable chunk of that on recruit packs. Aside from speedruns, it's easy enough to build up a decent amount of credits just by playing the game normally.
Your last paragraph touches on what is arguably one of the most important aspects of this game - rationing. When you're spending such huge quantities of credits on buying recruit packs (in my case, I've spent hundreds of millions of credits on recruits alone), it's definitely going to have a big impact on how you use your resources. You have to be wise with your limited resources. I've played thousands of games without using valuable items like cyclonic 4s. You have to learn to play with level 3 ammo bonuses while being content with a level 1 cyclonic and a level 1 weapon amp, that's pretty much a standard loadout for me. In my most desperate and tight times, I had to play some gold games with level 1 resources across the board...that was not fun. In any case, you're never going to be able to use max level consumables for every game as long as you continue to buy recruit packs to boost N7.
Cheers.