When should you buy a new tank? This might sound like a stupid question, but it is really not. When grinding (opening modules like tracks, engines and guns) lower tiers from 1 to 3, it is not important. Play a few games, unlock everything and move on. I would recommend that you unlock every module, though. That is because many modules are used on different tanks.
Let’s have a look at the french medium tank D2 (click for larger image):
French medium tank D2
As we see, it is only necessary to unlock the Renault S6Bis engine to unlock the next tank in this line, the B1. However, this will make the grind of later tanks more difficult, because the gun and the radio are used on other tanks:
The french medium tank D2
The radio ER53 is used on several other tanks. It is therefore a good idea to unlock it on the D2. (If you have not allready unlocked it on other tanks, like the UE 57.)
The same goes for the gun:
The french medium tank D2
As we see, the SA37 is also used on the B1. Even though you have to unlock the D2 APX 4 turret to use it, it will make it much easier to play the B1 if you have unlocked the SA37.
Therefore: You should always unlock all modules on a tank before moving to the next. (There are exceptions from this rule, when a module is used only on one tank. These exceptions are very few, though.) You should at least make this a rule from tier 3 or 4 and upwards.
The other thing you should be aware of, is the skills of your crew. As I have described, you have to get to 100% basic (or “base”) skills before you can start to add secondary skills on your crew. If you are patient, and play a few games after you have unlocked all modules and the next tank (or the next tanks), you can start to add secondary skills on the crew you will use on the next tank.
When I started playing World of tanks, I was not aware of this fact, unlocked and started using the next tank with a completely new crew. The old crew was dismissed, because I didn’t want to pay to retrain the crew to a different tank. BIG mistake. So wait a few games, and increase your crews skills before you move on. But remember to check the “Accelerate crew training” box when you do so. Almost all you XP will then be used on crew skills:
Accelerate crew training