Leveling really depends on your skill level and gears, and amount of time you're willing to put in to it. If you're playing for fun, and playing maybe 5 hours a week, might take you 5 months to get to high lvl trans.
If you're playing by grinding away in the most exp-given maps, and spending more time, you'd probably make it to trans in one or two months. People who give it unnatural amounts of time get there within a few weeks >_>
Rates are 5/5/3 btw from the original RO (ie. Cards in this game drop at the rate of 0.03%)
I dont know if there is a least popular class. There's definitely a most popular, and sinx and champ are high on there. Gunslingers, Ninja's, and Soul Linkers are not terribly numerous, far as I can tell, simply because people prefer a class that can trans. For classes that do trans, I believe the rarest would be sage/Professor and Bard(or Gypsy)/Clown.
And yes, to trans you start over at level 1 - but you get 100 extra stat points to allocate when you're reborn. And its harder to level a trans than it is to level non-trans.
Any more questions?