The 3g hotspot thing just means that (for extra $$) you can turn your phone into its own wireless router so you could connect a friend/iPod touch/laptop as long as you don't mind 3G speeds. Mostly I've found it to be pretty useless when I had the feature, but some people like it.
Check with the people at the store. Last I was in they swore you'd get notified if you were close to your cap and then could adjust your plan. I bought unlimited so I dunno.
Also, Android do or die unless you're a big Apple fan (but since you had a BB you're probably not
)
As for my usage I download quite a bit, but don't tend to stream music and use wifi as much as possible and despite my unlimited data (rather keep it than ditch it in case netflix comes through!) I have used .366 GB in 13/31 days.