I don't know much about home theater pc, but first, you should look into : HTPC / Media Center Cases. Like these :
http://www.newegg.com/HTPC-Media-Center-Cases/SubCategory/ID-690 if you want it to fit under your T.V., since Mini ITX cases are too big to be put in a desk like a DVD player.
You should be able to put in a regular ATX power supply, who would need between 500 to 600W for your need I would say. Any motherboards should fit, or according to what kind of mobo the case supports. The only problem, in my opinion, would be the GPU. You say you want to run Steam in Home Streaming and Blue Ray. Do you plan on watching movies and TV or gaming too? If you don't play on gaming, you should be able to fit in such a case smaller GPU but powerful enough for your need. Maybe some low profile GPU such as this one : http://www.newegg.ca/Product/Product.aspx?Item=N82E16814202091. It all depends on what kind of activity you want to do, and you need a GPU good enough to work well on higher resolution such as on a television.
Hope this helps!