No you don't have to buy stuff. You can play the game for free. I have not spent a penny on it. However if you want to, you can. It's faster to the better tanks and better equipment. Not a necessity. Been a lot of updates since it first came out and it's competitive even with free stuff. Yeah the C-Card commandos can be a pain, but you can overcome it.
I've heard a lot of this and world of shotguns, I assume they're from the same company. Now that I've got my new rig running, I might check one of them out.
For those still playing, there is a bonus code celebrating their 2nd anniversary.
Go to WOT website, log in, enter: 2NDBDAYTANKS under the redeem tab and you get a free 24 hours of premium account and a bunch of consumables (which you can sell for in-game credits).
If you have not played in a while, be prepared for a long download as it will have to update to the latest edition.