I wouldn't want a high mileage car, i have seen Z3m's for around £11k with around 80k on the clock but they are a bit untidy. you have made me think, should i spend the extra and go for the ultimate Z3 -- The M.
80k is not high mileage for a Z3! In fact it's very modest mileage. I think I can safely say most people on here would fall over themselves for an 80k mile Z3, especially a Z3M at £11k. Untidy is untidy though, regardless of mileage.
My own line in the sand to call something high(er) mileage is 110k. And even then if it's been maintained well there is nothing to fear and some screaming bargains. Just be aware chucking money at a high mileage car is unlikely to come back to you when you sell it.
The best driving 2.2 I've known is at 135k. And the best driving 3.0 was 130k. Those 2 cars drive beautifully, sound terrific and go like the clappers knocking spots off lower mileage cars. That 2.2 (Sport) is on the verge of finding a new owner for the princely sum of £2k. And it's lovely all round, not just the engine. If that was an 80k mile car I would want £4k+ for it. And honestly, it would be £2k that someone really doesn't need to spend if they just want a nice car to own and drive.
Of course if you have an eye on rare/investment that's different. But my personal advice would be it's a very risky game to play. Better to just find a car you like and pay sensible money.
You could spend £12-15k on a Z3M and end up with the same multi-thousand maintenance bill as an £18-20k car. That said £11k for an 80k mile Z3M sounds a little suspicious to me (might be a true bargain of course). I've seen them as low as £10k and it was almost too cheap to be credible.