Hmmm lovely engine work there, but agree with some others on here will do well to get £16k-£17k for it - at the end of the day the car itself is old, you could be looking at a newer Z4M, a better equipped M5, a variety of AMG's, RS4 or even go for something uber rare like a Lotus Europa all with equivalent performance. What makes the Zed such a great purchase at around £8k is what you get for the money at that price, there are not many other cars that come close, or looks wise have aged so well.
That said, you never know, I priced my car up at a premium and was told by a lot of people on here I wouldn't get the money I was asking for it - within a week someone came along who happened to be looking for a black Zed with all the stuff done to it that I had done and he paid straight up for it. You got to be lucky though