Hey,
I'm not really sure about the population but there are, more or less, 25 people playing. BGs never pop. People are mostly online at nights (If you live in Europe.) But all other activities like raids, hard mode astral sectors, dominions, AS are being done. Even though the population is not high, there are people who are very fun to play together (on Imp side, I don't have any idea about lig. But i'm sure there are not as cool as Imp.

)
Levelling up is really depends on you. If you have time and are determined, you can level up to 75 in one day. Actually, levelling up is not the problem, the problem is the important quests which you need them done in order to unlock maps, raids, etc.
If you ask me you can get game time and try it out for 1 month. If you like it, you'll keep playing. Or vice versa. 10€ don't make anyone poor after all.