Flareon 100% IV

No, not in my opinion.
It is so low lvl, that the amount of stardust and candies you need to invest in order to max it out is huge. You'll most likely evolve a new one from 600+ cp eevee, and even if the IV% is not that great it will still be good. Remember that IV only matters if you are going to power it up to the max level - and I find many better uses for stardust than flareon.

Definitely not. Flareon is a really weak defender. Most of your Eevee candy should go to Vaporeons if you need to power them up. However, I recommend you to be very conservative with your stardust usage.

I understand the urge to get high level Pokémon, but at level 24 you are still leveling up relatively fast and you will come by better Pokémon on your way to level 30. And the truth is, even if you power it up, it would still get taken down by a Vaporeon third its CP.