If you're not planning to get higher than lvl 30, there's literally no point in caring about IVs, as they only increase the maximum potential level of your Pokemon (which you'll only see at lvl 40).
Not true, IVs will affect how good the result is with any maxed-out pokemon at any level.
Lets say you are level 26, you have two Gyrados maxed out, both with good movesets, then the one with the better IVs will be better.
At close to 800 candy, just evolve the best IV 80%+ (top category in appraisal feature) that you have. Movesets are also important and they are random, and you will not have regrets if you find a better magikarp one day later, because you are so damn close to evolving a second one.
I would not evolve anything below 80% IV, because Gyrados (with a decent moveset) is actually worth the stardust to power it up to the max.
By the way, the IVs are not depending on your trainer level, only the level of the pokemon are. If a snorlax spawns and 30 people catch it, all 30 people will catch a snorlax with the same IVs and the same size in terms of weight/XL/XS. The level will be random, and your trainer level (up to level 30) determines the maximum level of the snorlax that you can get. So if it happens to be a 100% IV snorlax, everyone will get a 100% IV snorlax, but for some players it will be weak, and the high level players have a chance to get a real monster, and the low level players have no chance at getting a strong one, but everyone gets a 100% IV snorlax that has the potential to be a monster, if it has a good moveset and they invest whatever it takes to power it up.
You can justify powering up pokemon even when your trainer level is low, lets say between 20 and 25, if 3 conditions are fulfilled: High IV, good moveset, and when it is a pokemon that is good for competitive play. (No ratticate or pidgeot).
You cannot justify putting dust into a pokemon with bad IVs or a bad moveset, no matter what your player level is, no matter how strong it looks right now on the surface.
When any of the pokemon that have the potential to be awesome in arena enters your collection, and it has 100% IV, and you evolve it, and it gets a good moveset, but unfortunately it is initially weak due to low level, you need all your stardust to power up that one. And that will happen. It may never happen for a specific one of those that can be good, but with all the different pokemon that have the potential to be awesome, it will happen for some of them.
To sum it all up into the most important lesson: Play with the mindset of a very high level player, make your decisions accordingly, and you will have no regrets when you finally reach that very high level.
