Hey @bklofas ! Welcome to the forums 
Based on some testing we have done locally you can expect a fully uploaded observation to be no more than 35MB each. Interestingly it is the sound (ogg files) in most cases that make the bulk amount of those 35MB (around 25-30MB), and frankly it is the one that is less usable and unneeded in most cases. (i.e. we should consider having a low-bandwidth option in satnogs-client to only upload Waterfall and Decoded-data, and maybe CW and FMN transmissions?). Then as @KD9KCK points out, it really depends on how many observations you will have per day. For a UHF station of almost full utilization we are seeing up to ~60 per day. That said, there is no need to utilize it to such extend and a simple note in the descirption of the GS would let the observers know that this is a bandwidth sensitive station.
For OS and updates you would not need more than 100MB per month, but this is not based on actual observed stats. I will make sure to install vnstat in one of my clients to monitor actual usage.
Lately there have been more and more requests for facilitating low-bandwidth stations. We should consider implementation of a set of features (both in client and network) to make it easier for such stations to participate.
Out of curiosity: Where would that location be?