Welcome the forums! I bought a bit of a fixer upper myself so I understand where you're coming from with that.
Honestly though, the rough idle could be anything. I would suggest searching for user "jfoj" and clicking the links in his signature. He has a whole list of things that could be related to the idle. As for inside the intake, mine looked similar and I have no performance issues so I wouldn't worry too much about it. Curious, do you know if anything on the cooling system has been replaced? At that mileage, you're looking at the least a water pump, expansion tank, and thermostat. It's very easy DIY. I'm not a mechanic and I never worked on my previous cars, but these are very easy to work on and the parts aren't that expensive either.
Good luck! Hope you enjoy the car!
2000 323i | Orientblau | 160k miles