Anything that I say assume no Premium account (extra 50% credits and experience per battle).
There are 10 tiers of tanks. Tier 5 is supposed to be the sweet spot to earn credits, but my experience tells me that Tier 6-7 are better at making credits.
Tier 8 is my personal sweet spot because if you perform well, you make a bit of credits, not enough for consistent credit grinding, but more than enough to cover your running costs. If you play badly, you lose credits. Tier 9-10 are supposed to make zero credits and in most cases credit losses but they are primarily used in Clan Wars (consider it the end-game content).
There are two "pay-to-win" related complaints:
1. Matchmaking system that allows mid tier tanks to be in a high tier battles. For example, a KV-2 at Tier 6 has a chance of being placed in a Tier 9 battle. This makes it hard for mid tier players to tier up; and
2. Tier 8 Premium Tanks. This allows whoever buys them to go to Tier 8 straight away.
However, on the positive side, if you play without a Tier 8 Premium Tank, by the time you get to your first Tier 8, you'd have accumulated enough experience (literally, not "experience points" in-game) to deal with most of the tanks you have faced before. Also those Tier 8 Premium Tanks driven by bad players are easy pray to well-played Tier 6 tanks upwards.
The game is pretty grindy, because the only progression is to "tier up" (upgrade). There are no "sidegrades" like MechWarrior Online, but I have kept most of my fully-researched tanks just because they are fun. And the charm of the game is with a click of a button, you go into the battle. It's best played with friends in a Platoon.
If you need any help, let us know. There are some really good old-time players here like Rex_Nex. I personally specialise in French and German medium tanks.