Module and library for self healing large sensor network
A few thoughts:


For ~$20 per unit, the 1mW Digimesh modules will take care of reliable mesh networking plus provide a high degree of configurability and scalability. Depending on the budget for your sensor nodes this might be better than "wasting a ton of time" messing with bare-bones hardware and third party code.
Constrain the mesh network within the "monitoring area" and set up dedicated point-to-point links (with higher power and directional antennae) between it and your office. Set up two or three if you want redundancy.
If you can't afford better hardware but have time to spare, consider reading up on mesh protocols and implementing one you think best fits your project. Many of them are reasonably simple if you are not pushing for maximum efficiency or performance.