How long does it take to get 100$ from mining ? With 7/24 working. Assume electricty is free.
Difficulty is dropping and I think it's safe to assume it isn't going to keep trending up at 10-15% per month untill gen 3 and 4 asic's come online. So 4 - 6 months before we see difficulty growth if the price stays where it is.
That said you are better off buying new asic's than second hand ones.
Check out this calculator play with it untill you understand all variables. Put in you estimate for Bitcoin difficulty adjustments try 0 if you think it will go down.
https://bitcoinwisdom.com/bitcoin/calculator