sharp low light 1st dslr: a57 vs 600d vs d5100 vs a580

If you do not want to use the viewfinder anyway I do not think a dslr would be the best choice for you.

I think you would rather be better served by either a large sensor compact (like canon g1x) or by a mirrorless system like M43 (olympus,panasonic) or like Sony Nex or why not like Nikon 1.

The 50mm lens on aps-c I would say it will not be so good for indoor group shots especially in small spaces. Rather a 30..35mm on an aps-c sized sensor or 20..25 on m43 would be more appropriate. (The 50mm would be more appropriate for portraits, tho)