Josh A3

Hi all, I purchased a 2008 Audi A3 S Line 2.0TDi 170ps Quattro around three weeks ago and I seem to be having a few issue with power, smoke, smell and a increase in fuel consumption. Where do I start, all was fine when I took the vehicle out for a test drive, didn't really notice any problems until I actually purchased the vehicle. Since purchasing the vehicle I've noticed a horrible toxic like smell coming from the exhaust and the occasional cloud of white smoke (a bit like a cold start in winter but thicker and smelly). Along with that I've noticed that there is a noticeable power loss when accelerating and if cruising at a constant speed as well as a bit of a increase in fuel consumption. The gentleman that I bought the vehicle off of had the fuel, air and pollen filter changed a day before the pickup and said that the mechanic that worked on the car said that there was nothing wrong with it, E.I. brakes (Which I had to get replaced all round (pads, disks and calipers), turbo (makes a reasonably loud whistle and a puff of smoke when opening the bonnet and pressing the throttle), DPF (which was completely blocked and had to have cleaned along with a carbon clean)). So overall he said there was nothing wrong... Now, I've had a look around some forums for similar situations and I've found quite a few possibilities, some people have said it is the turbo on it's was out, or it's the glow plugs playing up etc etc... I would like to invite anyone that has had or experienced similar situations to reply/contact me as I'm approaching my wits end now! King regards!