to design a network

Tall order!
1) you need to carefully lay your ground work. You need to take into consideration how far workstation/server nodes are going to be from each other to determine the optimum connection method required.
2) armed with distance, connection criteria, you can start to choose assignment of hubs,switches and routers for each of the required topologies.
3) Since you are imploying DSL/ISDN I will presume internet capable networking so take into consideration that ip addresses done statically must be 192.168.0.1 through 192.168,255.254 or 10.0.0.1 through 10.255.255.254 so the whole network cannot be class 'c' if handling 5000 nodes . Also each router, switch, bridge constitues a node too! So you may be dealing with many more than 5000 nodes.

rick

maybe a bit late to come in with guidelines.

What kind of data is coming from site A - B
VOIP, SQL, Replication - site A-B-C-D fully meshed i figure, Mail server (Central ??), File server (Central ? or local ?).
It is not a question on how many routers (Which routers to by model for connection type Bandwith, E1, E3, DSL). Redundancy full loadbalanced, security, DMZ, Intranet/Extranet, who should access what 3rd party business partner.

Is there any way to get the Token Ring services over on Ethernet III, ease of troubleshooting only one Network Topology

Just to give a hint in what direction to go..