Hi Daniel, Judging from my pi3 mid relay where 5mbit traffic is about 20% of 1 core (tor uses 1 core mostly) 20-25mbit should be fine for pi3. In the peak hours it will be probably close to maximum what pi3 is capable on 1 core but when cost is a factor I would not think of anything else 1gb ram is more than enough, my tor uses about 150mb If you think of exit, cpu load may be higher