Check your local laws and/or muffler shops and see what they state. I just replaced my original factory cat. converter, today. It was clogged due to the biscuit working it's way loose and blocking the outlet. So I installed a Magnaflow. Car drives so much better!
If you're in California, and you plan on replacing the converter, do it before the end of the year. The state is rewriting their laws on those and replacing them. Don't know if it's going to affect the older cars like ours. Guess we'll find out after New Year's.
Since my car's mostly original and stock, I chose to leave the system with a converter. On my other cars, I've rebuilt the engines, and had fully custom larger exhaust systems made up. My '82 320i is a great little car and I'm gonna leave it mostly factory. Only future plans would be to add European bumpers and valances.
Good luck with your venture.
1982 320i 5spd.
1987 Volvo 245DL Estate 5spd.
1967 Volkswagen Beetle 4spd.
Too many surfboards & skateboards to count.