Oh but if you are wanting to go for an MP3 player then I might be able to help a bit.
An iPod is just a brand of MP3 player. They are known for being "the best" but TBH if you are just playing your music through a stereo, then you don't need an iPod. Also, if you use an iPod then you can only get your music via iTunes (or copying CDs onto the computer) - with any other MP3 player, you can use various programs to put the music on them, or you can even just do it through Windows on your computer.
iPods also have different connections to other MP3 players, so something which is iPod compatible may not be compatible with a normal MP3 player and vice versa.
Capacity just means how many songs they will hold. Most of them hold loads now, thousands. To be honest even if you went for a cheap one with a low capacity, you could plug it into the computer and change the songs on it before plugging it into your stereo, but this would be a bit annoying to do every time.
However if you get one with a high capacity then make sure it has a big screen and an easy to use menu or you will get annoyed every time you try to select a different CD or playlist.
The USB turntables are quite good and you can copy the tracks to CD or to the computer, you can then play them through the computer or put them onto an MP3 player/iPod.