I would swap over the interior from the 2.0 to the 2.8.
Possibly the facelift headlights too if you like the chrome ring look.
Sourcing a sports interior as good as that must be over 1k in parts, dye and not to mention time.
Then sell the HT and fitting just separately (sprayed in the colour of the buyers choice, say 1k ish)
Possibly sell the Z4 wheels separately but maybe leave them on as it will make for a quicker sale. Then put it up for £1,995
Adding the hardtop sale and the interior value and you get your 4K + value.