Use a good routing protocol (EIGRP or OSPF) in a robust design that features physical diversity of your WAN links (so that one backhoe can't get both wires). Be aware that there are enough requirements to achieve real HA to fill a book. I wrote one of them myself, you'll find it difficult reading that will put you to sleep, but what you learn will keep you awake at nights... good network designers are not paranoid only because all their fears are real, not imaginary :-)
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
