Z4's have huge market support right now, did you consider buying it brand new?
Trunk money for Aug 07:
Z4 3.0i roadster: $5,500
Z4 3.0si roadster: $6,500
Z4 3.0si coupe: $5,000
M Roadster: $5,000
M Coupe: $5,000
And the different between MSRP and invoice is about 3500, with the dismal sales I think u can get it easily for $1000 above invoice and take all the trunk money.
I picked up a new 06 3.0Si back in march for 12K off MSRP will a little haggling. 06 M roadsters are going for 15K off MSRP.
I suspect 07 will have similar discounts as the year rolls on, especially since the 08 is the sunset year for the Z4.
Also if you haven't signed the papers I would look into a lease return Z4, these things have horrible depreciation.