Hi there Gary, 1st i'd like to welcome you Lexus IS family, for 19 inch Wheel there are Pro and Cons option :
Pro : 235/35/19 FRONT - 265/30/19 REAR "You might have a little rub at the rear wheel if you carry more than 4 persons when you go over a pothole or a bump if your car is LOWERED like an inch" technically you're fine"
CONS : 225/35/19 FRONT - 255/30/19 REAR " This setup is 99% Closed to OEM SPECS its fine weather you lowered or not, pothole or bump"
and for the OFFSET, offset is up to the taste of the driver small or big Wheel lip in or out really up to you.
Pro : +35 Front and +38 REAR "this offset will look very nice and cool but i afraid the Rubbing when you make a sharp or corner turn
CONS: +38 Front and +40 REAR " this one is perfect without issue "
Remember our car OEM suspension setup is STAGGER , the front wheels are smaller than the rear by tire and width but Rim diametre are the same.
For Winter, let say you wanna save $$$$ and headache, i recommend you to run SQUARE setup Gary, remember the OLD Lexus IS250 AWD setup was 225/45/17 , So for winter storm ice and snow, it is best to used smaller wheel setup and our car can RUN WITH 225/45/17 on AWD, its will be alright and Tire's brand is up to the driver ? some likes Michellin some likes Yokomaha.