Securely erasing all data on iPod Classic so that it cannot be recovered
Since you're a Windows user I'm not entirely sure how the iPod classic will or won't work in Disk Mode or how your Windows formatting software of choice will see your iPod. However, on a Mac you can securely erase an iPod Classic, so it may well be possible with Windows.

However, in the absence of certainty, why not just fill it up with irrelevant data first? This will force a write over the storage.

For example, you could fill it with a whole bunch of music (or anything else really) and after it's full, then wipe it/restore it/etc. Then do with it as you wish in the knowledge that anyone seriously trying to recover the original data (i.e. the data you had on it before you filled it with irrelevant data) will have almost no chance of recovering anything meaningful.