iPod classic 5th generation

Good evening, I bought a "Sealed" iPod Classic 5th generation off of eBay. The seller has good reviews, and the images of the product are good, it has a 30-day return policy. I should be receiving it in May, I plan on upgrading it to the 3,000 mAh battery, and giving it flash storage instead of the 1.8-inch Toshiba hard drive it comes with.

I would like to know what I should expect. I already know it has to have files added to it, there is no WiFi or Bluetooth. And I know sometimes sealed means, it's been used and re-sealed. I want to know the important things, not stuff I already know.