The IVs mean almost nothing
IV means almost anything, the truly important thing is the level of the pokemon.
IV only means that at a SAME LEVEL the pokemon whit higher attack IV will deal slightly more damage, the one with higher defense IV receive slightly less damage and the one with higher stamina IV will stay slightly longer in combat.
Having said that, it will always be better bad iv higher lever pokemon than 100% iv lower level pokemon.
As it is impossible to always max out all your pokemons, it is preferable to capture pokemons at level 30 (ready to battle without spend stardust and saving it for the great ones) than pokemons with high IV
Answers
You are right.
Absolutly, 100%, ah, to a certain point.
It's obivously true that a pokemon at lvl 20 will deal less damage than it would at lvl 30. The differences from each lvl to the next are known and called CP Multipliers. They can be found as a list in the statistics menu right on the left. The menu i mean. You'll find it.
As you will find one of the recent threads were some smart dude explained what 'breakpoints' in the damagecalculation are and how they make a big difference in combat. Yeah, not only on paper.
I think it was the last one on this subject just a couple of days ago.
In the long long long run and in the big big big picture the higher the iv the better. In the short short short run and the small small small picture high lvl is better.
This game is not rocket science. There's not a lot of depth to it.
Obviously a 0%iv lvl 30 eevee evolved instantly into an espeon is quicker and wil have a higher cp than a 100% iv lvl 2 eevee evolved into an espeon. But in the long run....?
This game isn't algebra to the 5th degree multiplied by xy in a closed open circuit triangulated to netflix by the power of donald trumps hair stylist... candy to evolve an already high lvl mon without spending or collecting stardust (which member of the development team thought it was a good idea to call it stardust?) is easier and quicker than lvling up a 100% lvl 2 mon. Do what you want for what you need.
F^&* me with a crowbar...
This type of thinking/tactics is suitable for the mass in their level 20s and 30s - It helps them get battle ready and while conserving resources.
However, it is utter nonsense for top players (L39 - L40).
The problem is, If you keep playing, you will be among the tops, and eventually transfer these transient mon.
I use a mixed strategy. I try to develop high iv Mon, even if I have to spend dust on them, but for building raid squads of 6 of one kind, I will mix in high level wild catches with less than stellar iv. For example, my raikou counter-squad consists of 2 perfect rhydon that I have invested a lot of dust in, and four wild catches, which are "average" or "got my attention", and I only invested the candy to evolve them.
I figure I accumulate 75k dust a week, this is the amount required to power a raid trophy or hatch to thirty, or one Mon half the way from thirty to forty. In a year I can do about 52 such bumps.
I am going breadth first to develop a counter-squad for each relevant raid boss. Later, I will focus on a elite team of Mon that I max out. The latter will all be perfect or near perfect in IV , and few in number.
Why does it have to be one or the other?
If you think you have to choose between the different ways of doing it, then you are limiting yourself.
Just as laomashushu says above...a mix of both is optimal. Then you can have elite high IV mons, and finish off the attacking team with high level evolves.
You can change the level...you can't change the IV.
