It’s more than a month since you posted, and nobody’s replied. It also says, “inactive” below your name. Hmmm. Well, I’ve no idea if you’re still there, but reading your post I think your heart is in the right place for this music thing.
First off, the Yamaha 312B is a good choice and will serve you well. A comment often heard about Yamaha recorders is that they clog – fill with moisture from your breath – easily, but there are simple home treatments that keep that well under control.
If you haven’t purchased your instrument yet, I’d suggest buying yours from one of the traditional recorder-community vendors to avoid trouble. I’ve heard too many complaints about Amazon and eBay vendors that know nothing about recorders selling instruments with the wrong fingering and other such problems. The American Recorder Society website has a list of long-standing and knowledgeable vendors.
Aulos, Yamaha, and Zen On all make high-quality resin (plastic) altos that are recommended both for beginners and for long practice hours without some of the quirks of wood. All sound just fine. Resin recorders show up in public performances from time-to-time, which says something of their sound being good enough. I suggest to my students that they go with the Zen On. I can go into my reasoning if you wish, but you aren’t going wrong with these three makers.
Books that teach an instrument to a new player are called, “Method Books” as they provide a method to our madness, meaning that the book will teach you a note or three and then have some music using only those notes as well as similar practice material. Some methods teach reading music as they go along while others assume you have a teacher or prior instrumental experience. Some books have CDs or downloads that let you hear the lesson tunes. There are some excellent methods available that I can go into if you are still reading this website after so much time has gone by.
Another issue you raise: Alto (called “Treble” recorder in Europe) is an Early Music instrument. In part that’s why more music is out there for it than any other instrument such as guitar, violin, piano, etc. While children are often taught the smaller Soprano (Descant) recorder, Adults often enjoy the mellower lower notes of the Alto. The Treble Clef is the right hand of piano music and the one on which the music of many instruments is written. The lowest note on an alto recorder is an F, which is the lowest of the four spaces between the five lines you see in written music. To play something like “Misty Mountains,” you need printed music for the song that doesn’t go below that lowest space between the lines of the music staff (the five lines.) The Treble Recorder won’t play notes any lower than that. It’s not a big deal. Versions may be available in another ‘key’ that don’t go lower, or you can (with a little Music Theory knowledge) “transpose” the music to another key that doesn’t go below our lowest note. Make sense?
What you “Foresee” in your post is very reasonable and achievable. You sound like you’re in the right place for this musical journey. You can do it all without a teacher. I’ve found that my own learning is often much deeper on those things I’m self taught on. If you’re still out there, let us know how you’ve come along and what you need. A forum like this can be very helpful. Sound good?
Here if you need anything….