For a budget of only 500$, it should all go to suspension. Since you said you already have good tires (I hope you really meant that) I'd go buy one of the tokico shock/spring kits, and a poly bushing kit. Then look for swaybars.
Oops, that's about 850$. Well, at least it was easy! I wouldn't bother with anything else until you are done with that stuff. And if you don't have another car to drive every day, don't spend any of it. Keep it in a savings account for "when 30 year old parts go bad" days.
P.S. I will never believe the 240sx TB "adds 15 hp" to the stock engine without a whole lot of dyno runs that back it up. It doesn't bolt up with the stock linkage, either, you need a spacer.