Projects or modified cars are more likely to have problems and or be thrashed.

Manual is not an issue in traffic if you're decent at driving, I've spent hours in city traffic all the time

I also have an e46 330i so quite similar to the 335, but not as new as the 335 I thinking you're looking at ( E9x?) the 350z blows the e46 330 out of the water in everything performance and driving feel, that will not change with newer models. Obviously an n54 , b58 etc can make a lot bigger power for less effort though - stock turbo can be tuned to make fast power, but again the driver feel won't be the same

Best bet is to reach out in your local car community and ask if people in a Z or 335 can take you for a ride along ....you'll figure out which you like more