-a basic store is going to cost you 20.00 you will receive 200 BIN listing free each month. it would depend on how many item you have to list and whether it is viable.
-all your items will go into the store. you can't select what does and what doesn't.
-all listings will show in the store
-i don't think people care what item are in the store
-i wouldn't be starting a new ID if you don't have to.
- why not try a store for a month and see how it goes. you can always close it down.

http://pages.ebay.com.au/help/sell/storefees.html