You can put 225/40-18 front and 255/35-18 rear tyres on those 1-series wheels without a problem, making the tyre size the same as a stock Z4. The 2mm inset on the wheels compared with stock shouldn't be a problem, as 245 front / 275 rear tyre widths can fit in the wheel wells without rubbing.The Z4 wheels are: 18x8 ET47 / 18x8.5 ET50 (F/R)
while the 1 series are: 18x7.5 ET49 / 18x8.5 ET52 (F/R)
I would think the rears would be close enough to not be a problem, but the extra 2mm inset could conceivably cause some clearance issues unless you used spacers. The fronts look like they'd fit, but I'm not sure I'd want to stick a 1/2" narrower-than-stock wheel on the Z4. Even apart from the increased understeer tendency they'd look way inset in the front fender wells (again fixable with spacers).