AHH, MTBF date from vendors....well, there goes the idea of THAT project. You'll find that data, IF you can find it, will be calculated by sales cretins, not engineers.

Check out this book:

"High-Availability Network Fundamentals" Cisco Press ISBN 1-58713-017-3

Despite its Cisco Press origin, the book is 99% vendor-neutral and applies to any equipment. It helps you calculate MTBF-based availability of entire network paths, factoring in various types of redundancy. You're on your own collecting actual MTBF data from vendors, but this book may help you put it together into something sensible.