i'm 5'11" 180 and i wear a size 42 in alpinestars. i recommend going to one of the local shops and trying on the jackets in person. if you wear suits for work regularly it should be close to your standard business suit size.

i have:
-alpinestars textile air flo jacket. i went down wearing this and i will never wear a textile jacket again
-alpinestars smx air flo leather jacket. i love this thing and it fits perfectly
-alpinestars sm-x boots
-alpinestars sp gloves
-alpinestars tmx 1 piece suit
-hjc helmet that i got with the bike from the dealership
-agv gp-tech helmet
-alpinestars knee pads that i don't ever wear

so to answer your questions in order:
1. i prefer alpinestars gear all the way around. their textile jacket, although it melted on the pavement, probably saved my arm
2. try a 42-44 in jacket size. remember, anything that is dangling will catch if you're on the ground and rip open.
3. i bought most of my gear online. www.extremesupply.com has good helmet prices. i bought the other gear directly from alpinestars.com and ebay.com
4. for winter you should just layer up, make sure your jacket comes with a liner, wear some underarmour and definitely definitely get leather
5. ok