Ahhh a few of the NO2 myths are popping up here. As a long time user of the stuff here is what I know. (I used about 700hp worth of spray in a big block Dodge, WAY Fun but I would drain a tank in 2 to 3 runs).
Myth: N02 is cheap.
False. NO2 is cheaper to start out with, but in the long run, a supercharger or turbo is less expensive.
Myth: NO2 will wear out your engine
False.. Sorta. Any time you add horsepower to a stock engine you will tend to wear it out faster. But 50hp is 50hp no mater if it came from a turbo, supercharger, NO2, cams, FI, etc etc..
Myth: NO2 will blow your engine.
False. This one started back in NO2's cheater days. They (NASCAR) would find a way to dump the nitrous unmetered into the intake track, many times the extra 02 it put in there would lean an engine out so bad... BOOM.
After the first NO2 kits hit the market in the middle to late 70's they ended up on new stock Z28's, Trans-Ams, Corvettes etc. By this time the strength of the bottom end of the engines were far from the "glory days" of the 60's and they just could not stand the extra hp.
Some basic NO2 tips.
If you build the engine they same way you build a turbo engine good forged crank, rods, pistons, low compression, "bigger" cam on the exhaust side, larger exhaust system. you will make much more power than the hp rattings on most kits and the engine will last a long time.
One thing to keep an eye on is the fuel system. For a hard core system make sure you have a dedicated fuel system for the NO2, don't rely on the computer to send the extra fuel to the injectors. a clogged injector could put your engine into a dangerous lean condition and BOOM.
For a stock block find out how much added hp it can handle and use that as a guide to how much to spray.
Hope that helped