Happy to welcome you into the saxophone world. The mouthpiece is a critical piece of the instrument and I think especially so for a beginner. That being said, a Yamaha 4C mouthpiece is a decent mouthpiece to start learning on. Since its used and already has bite marks on the beak you should probably have it looked at by someone with experience playing the saxophone to make sure the tip and rails are in good shape and the mouthpiece is functioning properly. Still, you will probably want to move up to a better mouthpiece as soon as possible. In the affordable options, I would recommend looking at Vandoren AL4 or Selmer C* for concert playing and/or a Vandoren V16 series or D'Addario Select mouthpiece for jazz playing. All of these mouthpieces for the best prices found on line can be purchased at Saxquest at the link below.

http://www.saxquest.com/store/category/new-modern-sax-mouthpieces-C6/type/4

As for ligature, I wouold wait a bit on buying a high end ligature until your sound is a bit more developed and you can truly feel the differences between ligatures for yourself. A standard two screw ligature is fine for a beginner and even many professional use them. So, I wouldn't recommend spending any time worrying about ligs just yet. Time is better spent in the begenning in the practice room. Long tone excercises will get you a good sound at first much more than any equipment will so long as you have good functional stuff.

Yes, your saxophone is good enough. A YAS-23 is a decent student saxophone. But have it looked over at a competent shop to ensure its playing up to its potential.

Lastly, there is lots of good literature out there for saxophone. Greg Fishman has created several great books. They may be a bit advanced for a beginner but if you've played in the past they may be good for getting you back up to speed. Saxquest also has a list of listerature available at:

http://www.saxquest.com/store/category/sax-literature-C11

Cheers,
     Mark Overton
     www.saxquest.com