Seems like everywhere I've read people say you need to bypass the iPod dac to actually get the best sound
Line-out is what it's called. The iPods don't have it. If the amp section of the iPod was poorly designed (high output impedance for example), you'd benefit from an external amp even without bypassing the internal amp. The iPods from earlier generations have around 4 Ohm output impedances, but as long as it's not more than 1/8 of the headphone impedance, it should be fine. (rule of thumb)
I listen to rock and metal mostly, and that's all an amp will do is make them go louder? No help to sound quality? Hopefully I don't sound like a noob lol
It's probably going to be the only obvious difference.
Don't worry, we all started inexperienced and we still learn every day.
What would you recommend for headphones?
One thing you should know: never get headphones for an amp. Get an amp, if needed, for the headphones.
Portable headphones are designed to be driven easily by portable devices, so you often don't need to worry about lack of volume for example.
That mostly depends on your budget and the type of music you listen to.
Rock and metal and probly about 400 on headphones and then not sure what I'd spend on an amp
I honestly think that the type of music you like is much less relevant than the type of sound you like. Yes, those are two completely different things.
The type of sound you like can be dark (elevated bass), warm (elevated mids and maybe a bit of mid-bass), bright (elevated treble), V- or U-shaped (elevated bass ánd treble). Or like me, you might like a neutral sound signature that doesn't emphasize any frequency range.
You may also like a huge soundstage (like the open AKG headphones), or a more intimate soundstage (like most on-ear headphones), or a more balanced soundstage (like the Sennheiser HD600 or the Beyerdynamic DT-880).
What is it that you like yourself? No one other than yourself can tell. The best thing you can do is to listen to the headphones yourself to find out what you like. You can read reviews about the headphones that you like and get familiar with the terminologies.