Could you not just backup the data across the WAN to your central MA using client side deduplication without using a local MediaAgent at all? We backup customers all over the country via sometimes very poor internet links without issue using a Global DDB at the central site to cut down on traffic.
It works out almost the same as having a permanent MA onsite performing DashCopies back but will cost you on recovery time if you needed to recover in place at remote site. If you recover at central office then ship out replacement gear then that would be fine to backup direct.
Use a small desktop PC with a 3TB drive in it to send to site, you can fit it in a pelican case for easy shipping.
Use a USB drive for the seeding process and copy the DDB across to it before shipping back to your office, this is much slower and fiddly process though. This would only be needed for initial seeding once seeded you only need DashCopies to move updates.