Implementing periodic sensor data storage and GPRS transmission
It's a long time now! I don't know if you still have interest in these questions, but anyway.

I believe the first two questions do not have a single/objective answer. The answer strongly depends on your application.

For example, regarding the first question: Is your hardware battery powered? If yes, then you should calculate the time intervals having the power consumption in mind and how often you want to change batteries. If you are not concerned about the power consumption, then it mostly depends on what you want to achieve! You should maybe ask yourself, why do you record these data? Is it because you need to react to something (like an overvoltage or an overcurrent), in which case you should perhaps sample more often or just to keep history? It is also a matter of how much memory you have available, how often you could sample etc.

Regarding the second question, I suppose the questions you should ask yourself are whether you care about losing sampled data in case a power-out occurs between two transmissions and whether you have enough internal memory in the processor to keep the data between two transmissions.