Well theres nothing wrong with a used kit so long is its complete. The last thing you wanna do is buy a used kit and then get nickled and dimed to death tracking down any missing parts.
With regards to weather you should go with a supercharger or nitrous. I've done both. Nitrous will be cheaper initially however depending on how often you use it or how much your spraying it can get expensive over time. The supercharger is just the opposite and the advantage is that it is always on tap. I prefer the supercharger personally but both will get the job done.