I ran nitrous for quite a few years (12 if I recall correctly). When you're starting out, instead of throwing a bunch of money at it I would make sure you have the following:
1. Good ignition system with a high side rev limiter. This literally save my motor once when I broke a u-joint leaving on the juice.
1a. Good fuel system capable of supporting the highest level of HP that you think you will ever use (i.e. don't use a stock style pump, get a good electric pump...or a dedicated pump for N2O).
2. Cold plugs. I like NGK R5671A-8 for that level (150 shot), it has a non projected tip.
3. WOT switch along with a master arm switch. This is just to make sure the system is only on when you are at WOT...and only on when you want it on.
4. If you want to save a few bucks, just retard the timing manually...annoying I know, but will save a few $$ if you need to go on the cheap. I did this for MANY years, only took a couple minutes at the track.
5. Keep a close eye on your plugs and learn how to read them. They will warn you if something is off on the tune, and possibly avert catastrophe if you pay attention...detonation kills engines very quickly.
6. Use high octane fuel. This is another safety factor around the detonation issue.
All the other things mentioned by folks are nice too, but these are my "must have" things.
Have fun :)
