Hi Teekay,
Wheel offset is important.
It is a measurement relating to the distance the wheel is offset from the centre of the wheel bearing.
Changing this is not to be recommended; as it changes the clearances within the wheel arch, thence bearing load.
These clearances can, of course, be measured; but not while under full dynamic loads; cornering and braking especially.
Therefore, you need to know the wheel offset.
From memory, most Toyotas are 43mm. You must check this as my memory is not what is was!.
Also rolling radius is important for all the reasons above, and the accuracy of your odometer.
Ideally, the new rolling radius (and hence circumference) should be the same as your current one.
There are charts available to compute these measurements on the internet.
Some even identify changes in clearances within the wheel arch.
I doubt any of them include our dear old Alphies though.
All the above also applies when lowering (or raising) your vehicle ride height.