For me, the best music site is still to just go out and buy the CD and then rip it myself. That way I can choose the format & size.
As for best mp3 player? That's entering into holy war territory for a lot of people, so I'll just give you a list of things to consider:
1) Do you want a built in battery or a player that uses replaceable AA batteries?
2) Do you want a hard drive based player, or flash based?
3) How many songs do you want to hold? Less than a hundred? More than a thousand?
4) What file types do you want to play? All of them will play mp3, but what about wav, aac, wma, audible, ogg vorbis and so on.
5) Take a look at accesories available. How easily will it plug into your truck? What about arm bands, cases, speakers, fm transmitors and so on.
6) Do you need any of the extra features? fm radio? pictures? video? outlook synchronization?
7) What about size of the device? Would you be happy with burning mp3 files to a CD rom and using a CD player to listen to them, or do you want something smalller like an iPod Nano?
Hope that helps...
-Mark