Once you start reading in depth about audio gear, actually picking out what you want becomes extremely difficult because there is always something better (and more expensive) unless you're buying right at the very top. We've all been there and done that (and for the most part are still doing it
).
The headphones are responsible for 90% of the sound signature, so that is where you need to spend the most money, the DAC & amp make up the rest but more often than not actually sound quite similar. Obviously, as you move up the price scale the sound becomes more refined but the differences can be surprisingly small. With so much good quality, cheap electronics coming out of the far east these days, it means that very good sound can be had for very reasonable prices.
The first thing you need to decide on is your budget with an absolute maximum and go from there.