I understand the not wanting to sell it. The only way of sell mine is for a better one (so an M3). But considering you don't have the sunroof or leather maybe selling it an upgrading might be better. Do you still have your original parts? I still have my original lights just in case. Cause you can always sell them seperatley.
A new car is a new "project" in my eyes. A supercharger on a M3 or 328 would sound a lot better then on a 318. I'm sure theirs plenty of you tube videos on it. Their are a few people that have done the S52 swap and lots of information available about it.
I have the leather seats and although their hot in the summer the heated seats totally make up for it!