You should probably say what your budget is.

1) Depends on the outer shell. I use a Big Agnes Horsethief bag, the outer shell can take quite a bit of moisture and it just rolls off. Some UL bags, probably not so good.

2) Probably a bit of a crap shoot.

3) A good pad is important, regardless of what bag you're using. I use an air mattress and add a layer of blue foam when I expect cold weather.

4) Western Mountaineering, Marmot. I really like the Big Agnes bags, too, but they aren't as popular in this community.