Prestiging

It depends on your area and how strong Pokémon there are. But generally anything from 1000 CP up to 1600 CP.

320 CP Pidgey will become roughly 1200 CP Pidgeot.

Go to google and type evolution calculator and bookmark it. Type pidgey and the CP and then press calculate.

I typically don't power up or evolve specifically for prestiging. But it can't hurt to be thoughtful.

Would strive for a number of good prestigers at ~1000, ~1500 and ~2000. I have a bunch in those ranges, just by chance.

Depends on gyms in your area, but I see a lot of gyms with the lowest pokemon ranging from about 2000 to 2600 CP so my training pokemon I use most are in the 1000 to 1300 CP range. I have some a little higher and some a little lower. On the higher end, you will only need counters to pokemon that can reach 2800 CP+. On the low end, who knows what you might come across, but if the first pokemon in a gym is too low I will often ignore it and look to the next for selecting my CP.

https://pokeassistant.com/main/evolver?locale=en
With PokeAssistant's evolver tool, you can input the CP of a pokemon that you have to see an estimate of its evolved CP, or input the CP of an evolved pokemon you would like to see a range of lower evolution CPs that might get there.