You asked what kind of Adsense income per site, I've got sites that will do a dollar or two a day, others $40+. Nothing really spectacular on any of them. Here is what is important for building an average content site:
1) The niche. This is the biggest dividing line between earning a dollar a day and making $100. You want to find the right balance between competition and search volume. Your own knowledge also plays a big role here.
2) Content. You want pages optimized for all of the top overture keywords for the niche. Later get a forum or blog up to go after the long tail. (Although a blog adds a ton more work to a sit and wait content site.)
3) Backlinks. There are two huge variables here. a) your content, is it good? b) your niche, do other site owners like to give out links? (e.g. gaming sites, links are a piece of cake, medical sites, not so much.) I recommend getting the backlinks before placing ads.
Content sites are a great place to start. Get your sites launched as soon as possible, because domain age is going to be really important if you aren't a very good link baiter.