The monster that you'll benefit the most is very dependant on your server items price.
I personally would go for Kongras/Sibangs or Medusas/Serpent Spawns.
There's no such thing as best level range to pick it, it is though a very transversal Charm and, imho, not useful.
No, you certainly should not, 2k points is quite a lot for the added benefit.
Only creature products of the selected creature is affected.