I know what you mean with the Corsa, i've driven one before and it was painfully slow.
The Golf diesels are excellent cars, I own a Mark V 2008 Golf GT TDi 140PS. I do quite a lot of motorway milage to and from work and on average I achieve 50MPG and thats without driving economically and at 70 on the motorways. I think this is the sort of figure your'll be looking at yourself in realistic every day driving conditions.
With the price range you've given I think the best one to go for would be a 1.9 TDi Match, as the GT and above models are quite expensive and even the high milage, older ones costing from 8k and the good ones as much as 11k or more.
The match is a great all round car with the most value, Road tax is around £115, insurance group is reasonably low and you should beable to squeeze out an average of 55ishMPG. The match also comes with decent equipment usually, such as auto wipers/lights, cruise control. You should beable to find one with around 60,000 miles for 7k if you look around.
As for performance I can say the GT TDi (while being more powerful I know) feels very torquey and powerful, i've been the passenger in a 1.9 TDi and can say it felt very well built, smooth and like it had some power to give also. The turbo's on the Mark V's seem to be some what uneven in their power distribution however, giving low power at low revs and throwing you onto the back seats with ridiculous power at high revs. That would be my only complaint with them, but it's very minor and once you get into higher gears it has plenty of go and is lots of fun to drive.
About the 100k miles + idea. Golf diesels should theoretically last for 250,000 miles or more, so I don't see much of a problem going for one of these. Just make sure you give them a really thorough check as there may be parts needing to be replaced now or very soon. In which case it might be better to spend a little extra on one with less miles so you aren't forking out huge amounts of cash in the first few months. Saying that though you could have the same problem with a car on less miles. Diesels can take a lot more hammer but they are very expensive when they need to go into the garage.
