What's the best thing to look at, IV or level?
OhLookHeKickedTheBall IV is the important one because you can’t change it, whereas you can change the level by powering up.
I would always advise caution against evolving and/or powering up any Pokemon under Level 20, simply because of the candy and dust cost.
To take a Pokemon from Level 20 to Level 30, it costs 66 candies and 75,000 dust, but to take a Pokemon from Level 15 to Level 30, it costs 96,400 dust and 86 candies, so quite a bit more.
I personally don’t keep any Pokemon with an IV lower than 90% as I feel anything under is just far too low.
YMMV but anything below 82% is really getting low.
The difference between IV and Level is that IV is how good a Pokemon is, whereas Level is how much it has reached its potential (if you look at the arch you can see the higher the level the further along the arch it is). So yeah, IV is important because that cannot be changed, whereas the level can be.