Z4 is a nice car, but you'd kick yourself you didn't go for the Porsche for the same money.
Just buy sensibly. You could end up dropping £3k on one on its first proper service. I paid more for my 2001 Boxster S than I could have gotten one for, but every major thing that usually goes wrong with them had been done. All new wishbones, arms, rod ends, joints, fittings, suspension , rms, IMS, clutch, seals elsewhere, coil packs, manifolds, lots of tidying, air con rads, brackets and refurbishing. In all about £5k on bits was spent on mine, the chap did all the work and I paid just over £8k. £16k of receipts in total too.