You should know that z8games always fail. The game is a lil bit buggy. That means it could crash or stop loadding etc. Then you´d have to reinstall it.
Also it is a big advantage to know how to quick scope. I can tell you: Never waste your money on Black market. Well, I spinned like 50 crates on 1 fucking weapon and I didn´t win a shit. So yeah.
You get the most GP in Mutant Mode or Zombie Mode. And in my opinion Zombie Mode is the best mode.
Well many negative things but it is fun