Exchange basic questions..

We are planning to implement exchange server in our organization, at the moment we are running with third party mail store. We already have Active Directory with two windows 2003 R2 servers acting as DC & ADC each with 20 cals. We are planning buy Small Business Server 2003 R2 to use for exchange. My question are…

1. How many licences(CALs) we need to buy? If we want about 100 mail boxes(including aliases and groups)?

2. Does Standard SBS server will do our work?

Thanks..