After a week of choosing and trying out the hardware, a fully functional prototype is laid out on my work desk. In its current configuration, it contains an Iteaduino (a convenient Arduino UNO clone capable of both 3.3v and 5v logic) with a sensor shield full of carefully hand-braided 4-wire connectors leading to 5 HC-SR04 distance sensors, a temperature sensor and talking to a sim808 via software serial. Iteaduino controls the sim808 via AT commands and some extra pins such as power switch, reset and state signals. The module itself is sitting comfortably on a convenient breakout board by Itead studio which provides a battery charging circuit, antenna and mic connections.

On the power side of things, a li-ion battery is connected to the sim808 breakout to enable charging via a solar panel we have hooked to its micro USB port. To make things not reset, we had to power Iteaduino through its regulator (duh), so the battery also feeds a DC-DC step-up which then powers our Iteaduino safely.