An ECU remap is the first port of call and will give the single biggest gain with no other
mods at all. If you choose to make mechanical changes then you will ne the ECU re-mapped to take advantage of these.
Celtic Tuning is my recommendation.
They should be able to take your 2.0 litre diesel to over 160bhp with ease. The 318d engine is virtually identical to the 320d from the same model range.
If you wanted more torque then a higher spec turbo (which will require extensive work) along with a further remap will quite possibly see over 200bhp with a corresponding increase in torque.
We all talk bhp but we all drive torque.