Here's a wiki that will show you the amount of SR-series ever made :P
Nissan SR engine - Wikipedia, the free encyclopedia
And yeah, if you're new with these cars, take your time to get to know them. Improve on what you already have, without spending too much cash. That way, if you happen to break something (but I doubt it), you won't have much invested (especially since you're only 19). Yet.
But like someone said. Tuning a car gets addicting, especially if you have a need for speed.
Start small, with I/H/E. You can even keep using those parts when swapping to a new engine. You'll free up some power.
If you want more, and can afford it, then you can think about swapping and investing big bucks.
Remember:
Fast, reliable, cheap. Pick 2.