If Dratinis are rare where you are and you don't have any 10k eggs right now (to find a level 20 Dratini as well as get more candies), and you don't have other top tier Pokemon as your buddy right now (e.g. Lapras / Snorlax etc), then you could set your Dragonite as your buddy, and then power it up once you have enough candies to power it up to a decent level.
With 24 candies you could power it up to level 21, costing you 26400 dust, with 2037 CP and 113 HP. If that's high enough for you to use right now, then you could do it if you need a strong attacker.
Otherwise if it's not, then wait until you have enough candies to power it up to the level you want (using the CP calculator here: https://pokemongo.gamepress.gg/cpcalc#/ ), in case you're lucky enough to find another Dragonite in the wild that's got a higher level.
Don't worry about IVs so much on high tier / rare Pokemon. For common Pokemon you can because it's not hard to find another Pokemon in the wild that has a higher level and better IVs. Moveset is much more important, and DB/DP is really good, only second to DB/DC, and the DPS is very close.
For rare Pokemon, if you can get immediate use out of it, by the time time you find a better one, the gyms you'll have taken will make up for some of the investment cost.
If Dratinis are as rare as you say they are, then power up the Dragonite. It'll cost 86 candies to get it up to level 30 (after Pokemon reach level 30, CP gain per power up is reduced). 39 candies less than evolving a Dratini, which is a big difference if your main source of candies will come from walking your buddy, and even if you did somehow find a better Dratini, there's still a 2/3 chance that it ends up with a worse moveset for attacking.
