It's practically impossible to have it all ways with a Z3M. Either stump up £15k plus for a nice one. Or look out for something that's a bit tired and needs work for £8-£12k. You're very unlikely to find a nice car cheap. Any other Z3 model, maybe even a 3.0. But not a Z3M.
So a car like this is a good opportunity for someone prepared to roll their sleeves up. What's it worth?
Work backwards from it being all good but with the 134k miles on it. £12.5k maybe? I guessed at that and found this one (140k) to support that.
https://www.autotrader.co.uk/classified/advert/201707127320799
Valuing it then becomes much easier. How much work does it need? £1k? Then I don't think it's over-priced. £2k? You might want to haggle but it's still up at the right sort of money. £3-£4K. It's over-priced. And the more it needs spending on it perhaps the more you might under-estimate the actual cost.
This one went on open auction for £8,300 (134k). The seller had done a lot of work. But hadn't got as far as the respray and under-bonnet work it desperately needed. It doesn't look pretty and it was open auction. I'm not surprised it went for as little as it did but at fixed price this could have gone up at £9k.
http://www.ebay.co.uk/itm/BMW-Z3M-3-2-Roadster-Reg-08-09-2000-134k-miles-NO-RESERVE-/122653041053
You do need to price up the repairs carefully as a Z3M has some very expensive parts to replace. Engine obviously. If that was knackered or you wanted to do big miles in it then you'll have an issue regardless. £5k for a rebuild or decent mileage swap-in? But even things like front air ducting can be outrageous. £10 on a non-M second-hand. Well over £100 on a Z3M for a new one as you'll be unlikely to find it second-hand (and it would be expensive anyway).
If I had a £10k budget to spend on a Z3M this would be top of my list to take a look at. Hopefully it wouldn't need a full respray, the engine would be sound and it needed no more than £1-£2k spent on it. I'd try and it pick it up for £9k( it's sold before for this) and if my repair estimate proved right I'd be happy enough.