The catalytic converters were an issue on the '98s, probably why you have an aftermarket cat. There was an "extended warranty" that lasted until 10 years after the build date of the car.
If it is throwing the P0420 code it will not pass emissions in a lot of states and that means that if someone were to buy it they would need to sink another ~$1500
into it if they wanted to get an OEM cat from BMW to pass emissions.
Also, 182k is a lot of miles and that might be putting some people off. Do you have all the service records? It would be good to say that if you do have them.
Good luck with the sale!