IP/FIBER + ADSL backup. What equipment?

Hi,
We plan to have a 10MB Fiber connection to the internet and I would like to backup that with a ADSL Line.

Is there a router accepting both links in the same box?
We plan to have a firewall after the router. The firewall will also perform NAT.
If I must have one router for each link, how does load balancing work?
How NAT works.

Thanks fore your help