Relay satellite design

Regarding the design of the relay satellite we can distinguish the following subsystems:

Orbital and attitude control/propulsion:

Since the relay satellites are placed at the Lagrangian points there is only some need for orbit corections. Hence we will make use of ion-drives powered by solar power.
Ion drives have the best thrust-to-weight ratio of all propulsion systems and hence help to minimize total weight of the satellites and hence also the cost of the complete project.
If the ongoing mission of the Wilkinson Microwave Anisotropy Probe reveals that there is an bigger need of station keeping we could always put in conventional thrusters and fuel.

Power system:

The relay satellites will use solar cells as their main power source and employ batteries for back-up during periods where the solar cells are shadowed from the sun.
The total necessary power can be estimated to be the sum of transmit power (max. 100W without noise considerations for amplifiers), power for electronics (receiver, transmitter, control) and orbital and attitude control. The transmit power of 100W is only needed for satellite 1, the transmitter on the moon and sat.2 need less power.
A conservative estimate would be P_total = 200W
The sun generates about energy of about 1367 W/m². Using a conversion efficiency of η = 0.2 we get P_solar = 273W/m².
Taking into account that we are not always in sight of the sun we should use solar cells of approximately 1..2m².

Communication system:

For the transmission of the data the relay satellites will use a communication system that is discussed in more detail here.