Internet link with dynamic IP in complex routing

Greetings forumers,

I would like some advice about using Internet link with dynamically assigned IP address in a setup where multiple WAN links are already used. DHCP assigned IP addr is not what you call convenient when making various firewall, nat and mangle rules. I am thinking to use a separate router in between just to get DHCP IP from ISP and then set NAT up. This would work of course, but it wouldn’t be as transparent as I would like it to be. Perhaps some of you had some similar experience and know better ways?