2. no. But you need to get a data plan. You will probably only need BIS unless you are planning on buying a BES server for your 5 users. You can also investigate a hosted BES option, check with the host for their recommendations on the plans to choose
3.Not unless you are buying a BES
4.Yes, if you want tasks, calendar, contacts to sync.
6. Yes, but they will have had to come in after you configured the Blackberry (email is only retroactive on a BES, BIS service is only from configuration forward). You will also have to configure the Blackberry to never delete the emails (default is 30 days)
7. No, 5 mb limit on the newer Blackberries. Office documents may need a 3rd party app to fully manipulate (google dynoplex)
8. Not unless you are using a BES
9.No, but it will show in the message window with a little check mark next to it
10. only on BES
11. Only on BES or if you are using the DM to sync
The current Exchange config will allow for pop access to email. You will just have to configure each Blackberry BIS for email access through the web interface. Cingular is at https://bis.na.blackberry.com/html?brand=mycingular
These answers are simplified (very simplified). Good questions though. Users can get a pretty full email experience using the BIS. BES is wonderful for OTA (over the air) sync of calendar, tasks, etc but is not really necessary unless you have someone who doesn't do their own scheduling. It is necessary for CEO types who have a secretary sched their appointments for them and then they just look at the little screen on their hip and show up at the correct room...