In order to say what a “reasonable rate of return” is we would need to know the value of Zcash. It could be .000001 BTC or 1 BTC or 10 BTC? Impossible to know for now.
I have installed on a PC with less than 50GB of space free without issues. When mining you need about 1GB RAM per thread to run properly.
As for other hardware questions see this thread: https://forum.zcashcommunity.com/t/how-to-properly-mine-zcash-day-1/