I think in general you're on the right track. I'd like to see a tubular manifold instead of the log manifold, but money is the issue?
If you're really gunna ball out with the 4" exhaust and go all the way to the rear bumper with it, I don't think you should need any oval pipe. There is no place under there where oval pipe will help. Save the money and don't use any oval pipe. If your budget is the issue, what ever you do, don't try to use 3.5" pipe as it costs much more than 4" pipe I believe. That being said, I've ran a 4" exhaust before on a BMW and I'll say I wouldn't attempt it on a B14 unless I absolutely had to. If I wanted to push the limits I might do it, but oh so many people make the 400-500 WHP numbers with a 3" exhaust. Yah it would be easier and everything would be "better" with a 4" exhaust but you should really ask yourself if that's going to be worth it for you.
If you're really gunna ball out with the 4" exhaust and go all the way to the rear bumper with it, I don't think you should need any oval pipe. There is no place under there where oval pipe will help. Save the money and don't use any oval pipe. If your budget is the issue, what ever you do, don't try to use 3.5" pipe as it costs much more than 4" pipe I believe. That being said, I've ran a 4" exhaust before on a BMW and I'll say I wouldn't attempt it on a B14 unless I absolutely had to. If I wanted to push the limits I might do it, but oh so many people make the 400-500 WHP numbers with a 3" exhaust. Yah it would be easier and everything would be "better" with a 4" exhaust but you should really ask yourself if that's going to be worth it for you.