If you're going with custom wheels, your wheel widths are fine, but I would suggest more offset. For the front I would go at least et 30 wheels and at least 235 tires (the factory even puts a 255 on a 8.5 wheel for example)
Then for the rear, you can easily do et 35 wheels and a 265 tires... I'm running an et 35 wheel with a 275 tire and my car is lowered....
Here is my setup with H&R lowering springs for example...
Front - 19 x 8.5 et 35 w/10mm spacers (basically et 25) and 235 tires
Rear - 19 x 9.5 et 35 with 275/30 tires...
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2009 Z4 35i 6mt, SpeedTech 6266 Gen2 turbo kit, Tial WG, VRSF 7.5 race IC, ER charge pipe, Tial BOV, Radium Fuel cell w/ surge tank and twin Walbro 525 pumps, Radium FPR and return line, Bluetooth ethanol sensor, Phoenix intake manifold w/PI, JB4 tuner, PR coil packs, Custom oil cooler, ECS Radiator, Spec 3+clutch, Mfactory flywheel, single 3.5" into dual 3" exhaust, H&R springs, 19" TSW wheels, 235/19 & 275/19 RE11 tires