Depending on what direction you want to take the car I'd do one of three things...
Get some original sport seats- will look original but not give you masses of support IMO. However, they won't draw any attention and they're a nice subtle upgrade.
Go for some recaro pole position buckets- loads of support, light and a classic shape. Only downside is they look very obviously aftermarket in cloth with big white 'recaro' logos. That said, covers can be swapped at a later date if like me, you find the cost of leather ones a bit much to stomach all at once.
Get some recaro CS- basically the same seat as the BMW performance ones above and offer the best of Oem style and aftermarket support. Have to say those above are the nicest non original seats ive seen in a z4.
Personally I wouldnt bother with any of it and save the funds to put towards something faster but I understand about illogical sentimental attachment so I'll leave it at that!