What is the proper GPRS / GSM modem setup to work with 5 voltage sensors?
You could use a GPS tracker - here's one that has (amongst other features): -


Programmable logging and upload interval
4 x alarm inputs
2 x analog inputs [GS902]
2 x relay outputs
1 x low voltage alert


It doesn't give you the full 5 analogue inputs you need but it's a start.