I spotted a flaw in your design. The load-balancer need to be between the router and the ISP's, not as you've drawn it. The extra LAN port is also on the load-balancer for a future ISP addition. Notice that many switches are also capable of acting as a DHCP server (I know my Dell Powerconnect 2848 units are equipped thus) so you may not need the router at all. (link to Amazon, USD130-ish)
The router itself would handle both load balancing and routing. Why would I want a seperate router and load balancer?
It is easily possible on the likes of MikroTik for instance:
I very much need a router to replace my old one regardless.