Help please

Hi everyone, just bought my first ever ford and already in love with it. It's got problems though and I'd like some help please.

Fiesta Mk6 2008 1.4 tdci Zetec blue, 165,000 miles, poorly maintained.

So the car idles and runs very roughly, especially when cold. It struggles to start and feels as though it's misfiring until it's warm where it just runs roughly. The engine is very loud. I get intermittent engine system failure light, mostly in the first 30 minutes of running and not usually for very long. It also has a noticeable lag between pressing the throttle and the engine reacting.

I've checked for codes but none are on the system? There's maybe a faint smell of diesel after a long drive but that might just be because I'm trying so hard to see if I can smell one and the engine temps are fine according to the gauge, also the gauge says that my mpg is above average if anything. 

😕

 Does anyone know possibly what could be wrong?

I'm going to be servicing it this weekend or next week, changing oil and all filters.  Also I have a strange whirring sound from the driver's side of the engine after it's been running a while and continues when idling, which I've been informed is the water pump, so that's getting replaced, along with cam belt, next weekend. Do you think a service might solve my noisey and rough engine? Or is it something else like the injectors? 

thanks in advance for any help 🙂