I don't have Band Hero but I can speak for Guitar Hero.
1. Yes, it will appear in the game as any other song and will be under the downloaded songs tab also
2. You go to the store in game and purchase songs using Wii points. You pretty much click download (after purchasing). You can redownload songs that you previously purchased
3. It's in your main memory but you can move the song onto the SD card, it's a hassle though because you need to do it to each song individually
4. I don't remember exactly, I haven't played it in a while but I believe you can (or was it Rock Band, i can't remember). You could always download a free song to test it out too.
5. Yes, I have a 2gb Sandisk in my Wii. I would have upgraded but so far i'm not even close to filling it up yet
6. Rock Band 2 and Guitar Hero World Tour and up are compatible with each other i believe
Hope that helps!