IMO s15 is not worth as much as your frs.
I had a R33 skyline for 6 years and I can honestly tell you as legal as I had it, it was still illegal. and every time a cop pulled me over (which happened a lot because the wanted to check it out) its a long story that you have to have some backup in writing and even then you ALWAYS have the chance of them taking your car. You drive around a little scared pretty much all the time always wondering if your investment is going to be around next week. My insurance guy was cool so when I got hit and had 5k in damage, I got it taken care of. you might not be as lucky.
You can go buy a s15 for less than 15k anyway so its not worth a trade for a frs either way.
Most people selling JDM cars they want a lot.. but you can usually take it off their hands with cash for cheap because they know its almost impossible for you to get a loan for one and they want to just get rid of it.
Love the cars, a buddy of mine had a pearl white s15 for a while, I wonder if its the same one