The answer is two part:
1. Yes you can access the server externally provided you know what the ip address is changed too.
2. As for the Exchange part, the MX records in the ISP's DNS will need to be updated everytime your IP changes. Doubtfull that the Cable ISP will do that....so I would say no, Exchange will not funtion properly with a dynamic ip. See if the Cable company will sell you a Static ip....
Hewissa
MCSE, CCNA, CIW