Noob Here Need Help

hey guys looking to buy an ipod classic and the hell off spotify. Ive done a bit of research and saw the differences between 5.5 and 6/7. Ive also done enough research to know that this games a rabbit whole and would love any advice on what to look for in a classic. I noticed some posts that show a screen with a bunch of log numbers what is that? Also ive read that the 5.5 has a warmer sound than the 6/7 any real experience with that? Also looking to mod it for bluetooth and larger battery etc in the future any tips to help me find a good ipod thats not beat the fuck up would be awesome. Thanks guys!