Since you already have a defending Dragonite, I assume you'd want one for attacking.
The 49 CP Dragonite has no chance to have an attacking moveset, it's already got it's moves.
The Dratini that you evolve has a 50% chance to have an attacking moveset (Dragon Breath + any charge move), and if it doesn't, then you probably would have spent the same amount of candies powering up the 49 CP Dragonite, but it would also cost you stardust.
It would help if you gave more info such as the HP of the pokemon so we could work out the IVs as well as the level of it, because then you can work out exactly how many candies / stardust is required to power it up to a certain level.