Hi JOn,
ok, to start off, I LOVE my Stilo. I've had some corking cars over the years, and believe it or not I LOVE the Stilo more (only just), and here is why:
Looks stunning - I have the 3 door Sporting
Easy to upgrade - I fitted all leather
Drives smooth
brilliant engine, loads of power/torque, mines on 106k miles
mostly cheap/reasonable parts
great brakes, very effective
great interior, I love its design
I suppose I could go on but perhaps that's silly. Im not saying its the best car in the world, but what I am saying is it is a seriously underrated car and so they can be bought so cheap!
The loud clunk, as Davren says, really does depend. We cant possibly diagnose it BUT there are TWO main common failures on these. As Davren says, front drop links - very easy fix, I did mine when I bought it and fitted new links with the wheels still on! The other is the rear bushes. Those are expensive (labour) and you will be looking at around £300, maybe more, maybe less, but once done you wont be doing them again. Mine do knock a tiny little bit, but not enough for me to open my wallet - drive sensible over bumps, and there are no knocks, simple. As for haggling the seller, yes, tell him you know a bit about these cars and that its likely the rear bushes (you can see if you get down and look up towards the front of the rear wheels - BIG chunky cylindrical things both ends at an angle) and tell him that's about £350 because the whole axel has to come off to replace those and requires a special extractor to get the bushes out.
If it is suspension or shocks, then the costs are no more than any other car, and no more difficult to do - probably around £50 a side all in.
other common faults really does depend on the model you buy - sunroof is a common failure, air con has a common failure in the pressure sensor but that's easy fix. Check all 4 settings on the heating, and check the temp gauge gets to half way after a good drive - test drive should be minimum 30 mins and also look out for any smoke out the back when flooring the accelerator (make sure you do that as well). If the temp gauge doesn't move, or is so slow, then I have to say its a new thermostat. I had this issue, car worked fine, but never got to temp - I fitted a new thermostat and it was a PIG of a job - costs in a garage id say at least 2 hours labour and £70 for the part (82degree thermostat, not 88 if possible) fitting requires removal of the EGR -
EGR is another common problem, though that's the same for any diesel car. The solenoids can fail, or the egr can stick - options are to blank it, or buy a new one but if you aren't competent its a garage job - over £100 for the part complete and at least 1 hour labour. Cheap option is to blank it using three blanking plates.
LIGHTBULBS, I cant stress enough, ensure all light bulbs are working because changing them is a pain. The proper way to change them, is bumper off, remove the headlight, change the bulb. Also, ensure the head light adjusters work - access controls are on the panel to your right, up/down buttons - make sure BOTH headlamps go up and down - if stuck it will likely fail MOT if one is pointing up.
all other mechanicals id say are reasonably basic and easy to fix, and easy to spot if faulty. If you look in the guides section, then select Stilo, you will see a lot of the common issues and how to fix them. Don't let anything I have said put you off, but do use it as a guide if you don't want any problems that will cost you more. Go armed to the seller, and see what you can do with the price if you suspect any of the above. Inspect the car for as long as you can - take a torch, even in the day light, so you can see all engine components in the engine bay, and underneath the car - I know they are low, but get the torch in there and inspect all nooks and crannies. Look for oil leaks, coolant levels, brake fluid level etc. When you test drive it, floor that pedal, slam those brakes, if it doesn't feel right walk away.
best of luck sir, do enjoy shopping, don't let the above worry you - there are some great Stilo's out there for little money because a vast majority of people don't know what they are and most people go for Golfs in this class. This only helps buying power because the prices are so low when compared to an equivalent Golf or Focus - use that to your advantage.