Just sell it and buy a 535i, same car 208hp stock, easy 220 with a chip, more torque which works well with heavier cars.
Or buy a 540i for more power yet.
Unless you are going turbo, I don't think I would mod that car much. If it is an automatic I would definately sell it and find a better starting point.
Headers, chip and cams will bring the power up the rev range, and make a better number but it will probably cost you 2 grand if you do all the work. Like everybody is telling you, that money could be better spent. Good Luck
Current ride... '13 GTI Wolfsburg
BMW's from my past...
'92 535i/5 '89 325ix/5
'98 750iL '86 325(e)/5
'95 740iL '84 318i/5