You can catch lower level 30 Druddigon in Dragonspiral Tower.
you probably level them up and use them in Pokemon's league
Once, you have caught one, it's the same as any other Pokemon. Yes.
I am not completely sure as I caught Blitzle in its evolved form Zebstrika but I think it is level 26 as mine is level 27.
You can only get a Vanilluxe cannot be caught in Pokemon black and white, the only way to get it is to catch a Vanillite or Vanillish and level it up to obtain a Vanilluxe. (Or you can trade with somebody who has one)
Thundurus can be caught an white, Tornodus can be caught on Black. To catch these two Pokemon, use a Pokemon with Mean Look, like Cofagrigus, so it cant escape. They will both be at level 40. To get the other one, use Global Trade or trade with a friemd who has the other game, then go to the Abundant Shrine near Undella Bay, you will need waterfall to get there, When you get to the shrine with bothe Pokemon, Landorus will appear, it will be at Level 70
Klink evolves into Klang at level 38 on Pokemon Black.
futachimaru evolves at level 99 in Pokemon black and wight
krookodile evolves at level 40 in Pokemon black and white
Level 43
Level 5
Level 64
train it to level 100!