Ok everyone, sorry it took me so long to get this information!! Here is the following information : Offset for the fronts is 45.8 mm, and the rears 55.0 mm.
If looking for snow tires, here is what Lexus recommends for wheel size:
Overall Size : 18 x 7.5
Pitch Circle Diameter : 114.3 mm
Offset: + 40mm
Center Bore : 72.62 mm
225/45/R18XL for the front
255/45/R18XL for the rear
I know the following tires work fine :
Michelin Pilot Alpin PA2
Blizzak LM-18
These tires can be ordered thru the dealership !!