Welcome Alex, congrats on the new ride.
Not sure where you are located, but with regards to the exhaust, I may have something to offer. My mother had an E39 523i and while we had it, we took it to RGM (Rob Green Motorsport) for a Stage 1 tune up. This included a performance airfilter, chip and tuning and a full custom exhaust. This was done around 2007 if memory serves me correctly. However, when we sold the car, to an elderly lady, we fitted the stock exhaust back on and now have the custom RGM exhaust system sitting in storage in our rooftop.
All that I can recall is that it had the meanest and most wicked exhaust tone I'd ever heard out of a 6 cylinder E39.
Let me know