I've had my petrol just over 7 years now, bought it completely standard with 71k miles. It's a 97 R reg and cost £4400! Not a rough old banger either, not quite standard anymore. Simple answer to the mpg when I'm asked, it'll do upto 21mpg.
Anything below that is just what it is. I can easily get single figures and tunnels are for 2nd gear haha. Noise wise in stock form they're not noisy from what I remember (mine is a bit loud)
As already said have an idea of what you want and buy the first good one. They're not new vehicles anymore, the young ones are in the 20's.
Now personally I would not have a diesel as like the petrol and the performance it gives. Driven both when I was looking for mine. At the time I gave up looking for a diesel as wasn't willing to pay twice as much if not more for a not nice high miler.