BMW OEM 19" wheels are a staggered fitment, in that the rears and wider than the fronts.
As standard...
Front wheels are 8J x 19" (or 8" wide by 19" in diameter), ET37 offset
with 225/35 tyres
Rear wheels are 9J x 19" (or 9" wide by 19" in diameter), ET39 offset
with 255/30 tyres
Your ACS wheels have an offset of ET43, and are 8.5" wide. Given that they have a 6mm greater offset than the standard 19" BMW front wheels (ie sit 6mm closer to the hub), but are 0.5" wider (12.7mm - or 6.35mm either side of the centreline), then the outside edge of the front wheel will be only 0.35mm further out than normal. The inside edge of the wheel will be 12.35mm closer to the hub. This shouldn't be a problem!!
However, you may find that the rear wheels (if you are using the same wheels as the front) are too narrow - offset difference is 4mm (so the wheel sits 4mm closer to the hub than normal), and to compound that the ACS wheel is narrower by 0.5" (12.7mm) than the OEM BMW wheel. Therefore, the outside edge of the ACS rear wheel (assuming it's a 8.5" ET43 size) will be 10.35mm
further in than standard. Multiply the effect by two (both rear wheels) and your rear track is narrower by 20.7mmmm, or just under 1".
If you can, get yourself a 9J wheel for the rear, and use the 8.5J at the front.
See below for diagrams etc explaining offset measurement.
The offset, or ET measurement in millimetres, determines the position of the
mounting point of the wheel in relation to the
centreline of the wheel (see diagram below)
Looking at the picture above, note that the left hand side is the brake side, and the right hand side is the outer edge of the wheel.
In the left image, you can clearly see that the mounting point is positively offset to the right of the centreline of the wheel - so for instance an ET37 wheel will have the mounting point 37mm to the right of the centreline.
The middle image shows no offset at all, so the mounting point is in the same position as the centreline of the wheel.
The right image shows negative offset.
The higher the positive offset value, the more the wheels sits IN towards the hubs. The lower the offset value the closer to the wheelarch. In both cases, the width of the wheel is constant, the only thing that changes is the mounting position relative to the centreline of the wheel.