They kinda want you to spend money, or getting a decent tank with equipment will be a nightmare.
Urmm no.
You get more exp and credits per game with premium, but its not necessary at all.
"Decent tank" is also definately a thing of view.
Tier 5 can be really fun already, and you can reach that in like 2 days without premium.
Tier 7 is pretty much the tier where you will discover the full efficiency of the tank tree branch, and its perfectly reachable in like 1 Week.
The later tiers require more effort then.
Also, it seems to me that you have a wrong image in your mid of that game.Its not about standing in front of every enemy tank ingame with your tank and letting everything bounce on your armor.It requires tactics.
And no, the Tiger is not a joke tank.It´s pretty awesome because the long 8,8 has over 200 Penetration, which basically means you can hurt almost every tank till tier 10 in the Front.