My "other two cents":
What you contemplate is a lot of money for a used 328i.....and, it doesn't sound like you've locked yourself in to a car. Many of us commit to a car, thinking it's great, bone stock, and later decide to make changes to it to keep things interesting. In your case, it sounds like you haven't bought a car and you're laying out all the things you'll do after you buy it.
If it were me, I would look at the cost of 328i + 15k plus mods and think there are plenty of other cars in that price range (besides a 335i) that are likely superior to the 328i. Before committing to your plan, I'd be out testing a used Audi TTS, a used M3, a used Cayman, a used....