The USB standard specifies 500mA per port. (See
here, or full spec
here.) This is less than most 2.5" drives need for spinup. Many PCs actually will supply more than 500mA, but not all. Therefore some disks come with extra power cables or other solutions.
As for drive longevity and skip protection, I have an mp3 player with a 2.5" drive that I've been using for about four years now. The last year it has been in my arm rest, connected to my aux port (too old car for 6fl). It powers on whenever I start the car and turns off when the car is turned off, so it's pretty much the same situation as for 100mph. There has not ever been a single indication of problem with this disk.
Another thing that is good to know is that laptop drives are only vulnerable in the very short moments they are reading/writing from disk. Even when spinning, if no read/write is happening the heads are safely parked in a shock-resistant zone. Many disks use aggressive read-ahead caching that further minimize the time spent "on the platter".
Combine this with the fact that all MP3 players need to buffer data in memory to decode it, and you'll find that your disk is actually in shock-protection mode most of the time. (You only really
have to buffer a fraction of a second, but most implementations load far more into memory, because the gain far outweighs the little extra memory use.)
In short: You'll be fine!