You know, in your situation you might consider running 1 SEPM and use GUPs to distribute the updates.
My current setup is similar, we have three locations. I have the SEPM at the data center and setup a GUP in each location. The GUPs were setup on boxes that have Windows Server 2003 installed, so that there would not be a connection limit. Most of the bandwidth used is from update distribution, not policy updates.
The built in Sybase database should be fine in your case. The client limit is 5,000. I would make sure to make a backup of the DB and the keystore on a regular basis.
Mike