First, I don't own an iPod (yet...), so I can only answer some of your questions...
There's no direct line in on the iPod, but it might be possible to record via some adapter for the dock connector. But I'm pretty sure that it doesn't provide phantom power. That's 48V and only used for symetric microphones.
As the iPod can be used like an external harddisk (at least on a Mac) it can store any type of files, including video. But it cannot playback video, not through the analog 1/8 plug (which is audio only) and not through Firewire/USB. It can only playback audio.
I don't think that the file size has any limit, except the storage capacity. But I'm not sure if it will playback a single 3GB audio file.
I don't know if it works with Windows Media Player, but you can use iTunes for Windows. It's free, and I think it's even included with the iPod (but I'm not sure about that).
It supports following audio formats: AAC (16 to 320 Kbps), MP3 (32 to 320 Kbps), MP3 VBR, Audible, AIFF, Apple Lossless and WAV. Firmware is upgradable, so this might change in the future...
It uses a harddisk for storage, 4GB on the iPod mini and 20GB/40GB on the iPod. Some older generations have 5GB, 10GB or 15GB harddisks.
As I said, I don't own one, so I can't tell how much damage it can take. But I heard they're quite robust.