Why not request a static IP address for your business?
Then you can do away with the T1 line altogether. Poke a hole in your DSL router to forward port 25 to your internal exchange server.
Set up the DNS mx records for your domain to point to your static ip address.
If you can't go static, then it may be possible to use the dyndns service. I don't have any experience with dyndns personally as we've always been able to get a static address.
Yes and to answer your question directly. You will only want one DHCP server on the LAN. Most routers have it built in. I'd disable them all. Put DHCP on the windows server. Set a static ip for the server and set it's default route in the network card to be the T1 line. In the DHCP server make sure that the default route is the DSL router ip address.. Make sure that you set your routers with internal ip's that will never conflict with what the dhcp server will give out...usually I just set dhcp server range to be from 192.168.1.1 - 192.168.1.200 or similar. Then set your routers to be numbered above 200.
The reason for using the windows dhcp server is because it interoprates with windows dns. Windows DNS is pretty vital for computers using active directory. A built in dhcp server doesn't do and support all of Microsoft's non-standard extensions.