I would buy the cheapest base kit you can find. All the kits these days are really well built. I might stay away from Zex and Nos, but even they have decent kits.

I would buy as much stuff as you can afford. Nitrous is easy to get into, but that is where people have problems.

I've posted my setup before on here(don't feel like writing that essay again), but I have close to $1000 into my setup and it's not even very extensive.

As far as how big of a shot. The stock bottom end can easily handle a 150 if tuned right. The bigger problem will be your transmission. Even a window switch isn't going to help when you're adding almost 200 ft/lbs of torque(Our trucks make a ton of torque on nitrous).