I have owned a Shadow alto and I own a Selmer Reference alto. For jazz, I would pick this particular Selmer model, I think it's an incredible horn. The Shadow is also an excellent horn, by the way, but probably better for other styles.
Don't listen too much to people who put down a horn JUST because it's expensive. In my experience, most of the times, the more expensive horns and mouthpieces were in fact the best ones. There are exceptions, but this is the general trend.
As to the mouthpiece, that is an even more personal choice than the horn. Material doesn't matter, it will depend on how comfortable it feels for you but there is no actual impact of material on the sound. If you are just starting out, and if we're talking about alto here, I would recommend a Meyer. But I wouldn't buy a stock one, I would put in a little more cash (not much more) and order a custom Meyer from Phil Engleman (Phil-tone). Two other excellent possibilities are the Aaron Drake NY Jazz mouthpiece or the Mouthpiece Cafe Brothers New York model. These are a little more expensive than the Meyer but well worth the investment.
Of course, other people will suggest other mouthpieces and you will end up confused like I did when I started playing sax and asking for advice.
I actually have a Phil-tone Custom Meyer for sale but this is not the purpose of this post, I don't want to sound like I'm taking the opportunity to push it on you.
Above all, good luck!