there is almost no tuning potential in a 320i whatsoever. It'd cost you nearly £1000 just to get even a mildly noticeable improvement in power. A remap would be good for maybe about 5-6bhp, an exhaust maybe 1-3bhp. So theres £700 wasted!! im not convinced you could even feel the difference.
Either leave it as it is, or sell it and buy something more powerful / more tuneable.