1. I actually rather like the way the transmission reacts to throttle inputs. You won't see any downgrades here in comparison to other automatics. The thing to look out for on the RWD cars is the common reverse drum failure on ZF transmission.
2. On RWD vehicles the DSC can be partially disabled with one push of the "DSC" button and completely defeated with a long push
3. Other typical E46 issues to watch out for are rear subframe failure, cooling system (water pump, ET, etc.), and window regulators. Check for a full service history with the previous owner and have the car inspected before you buy so you know what you might be in for.
4. The 330i you are looking at is not an "M-sport" model. Those typically have different bumpers and style 135 wheels. The ZHP is the 330i with performance cams, making a little extra power. An M3 uses an engine with an entirely different block from the lesser models and puts out significantly more hp.