I am not sure, but I think no one is offering Blackberry services on PrePaid. All telcos are offering on PostPaid.
Blackberry devices without blackberry services are of no good. GPRS is already covered in Blackberry services, so you won't have to pay separate for GPRS usage.
There are different packages, where you can get 1-10 emails configured for your blackberry. It does include yahoo and gmail.
Best is to call their customer support or go to their service center and you'll get all the details.
Don't buy handset from telcos, as they sell at awfully high prices.