Ipods are meant to be portable. The only problem is that when you play a Classic in a car and there are joints in the road, the HDD will skip. That is impossible with any kind of flash memory (SSD included). That is the point of the original post.
iPods have buffering. There should be no skipping unless you're driving on railroad ties or cross-country.
Assuming that your library will fit, you would have fewer headaches if you get an iPod Touch or iPod Nano.
Right. The hard drive doesn't spin constantly, feeding pieces of the song as it plays. The device loads multiple songs into memory, spins down the hard drive, and pays through them. Then when the queue is getting near the end, spins it up and loads more songs.
The only hard drive jitter you have to worry about in this case is when it's loading more music. If you had such a jittery environment that it wasn't able to reload the queue before it emptied, you could get some skipping and other artifacts.