Grumpy Mike wrote the RadioHead library which would let you set up a low cost mesh. Aside from your "manager" nodes I would imagine that you would be able to power most of your hives with an inexpensive solar panel and small battery -- using the Arduino itself to do simple charge management.
With respect to the load cell you select make sure you have a credible way to mount it. A shear beam cell with a single point of contact wouldn't be practical unless you had the hive mounted on a pivot. Also, shear beam cells need rock-steady mount points (steel/concrete); if you try to screw the fixed end into wood I would expect it to start pulling its screws out and thus require constant recalibration. As an alternative you can get three-wire load cells (cheaper on eBay) like those that are used in bathroom scales, mount them in the corners of the hive, and would be able to wire the four of them together into a full bridge for a more practical mounting solution.