First be prepared for the mental stress.
Get thorough knowledge of the network existing.
Talk to service providers/vendors if that technology is available.
Get all the config steps ready jotted ur self on paper.
Try testing each command if its working.
Take 2 consequtive holidays and start working,if you are working on production setup.
Surely plan for most economic setup..bcoz finance shld have to approve it..but never compromise on redundancy..afterall u need to sleep and take rest.
Gimme some tips where u have many branches to a single router and how u can avoid single point of failure.