ok, thats a weird setup...
What you could do is just put on router on the dsl. (a firewall would be nice. if it is a business i would highly recommend it, if they have a spare desktop, put a second NIC card in there and load untangle firewall on it) Then you just need a switch and you dont need anymore routers.
Also they dont need to use more then one static unless they are planning to host something
If they are, then just configre the firewall as a static and all the 5 addresses to the config and then nat it from the public address to the privet address
(also i am being vague because i dont know what you want to do and i dont want to spend 30 pages telling everything you can do. i can assist more if you know what you want to do)