My IS250 sport with mm is a superb car and has provided no cause for complaint. It is fitted with 18" wide alloys and tyres, which look great.
Problem is, I believe that 17 inch wheels offer better refinement and lower road noise (is this true). Also, I want to take the car to the Balkans where driving through snow and ice is common for 3-4 months of the year.

Though I should have bought the SE with 17 inches, that sport looked great etc...

Questions:
(a) Is it as simple as buying and fitting 17 inch instead of 18", and then
( putting winter tyres on the 17"?

It would also be nice to have the suspension lifted a little further away from the ground - the sport model is lower slung towards the ground.

Any advice to make this car safer to drive around in winter conditions would be appreciated

Hi, I am from Slovakia and we have hard winters sometimes too

I have 17" for sommer and 16" for winter. I know, it looks funny from behind on 205/55/16 tires but I have no troubles on snow/ice even on hills - RWDs have problems with winter.
I use Pirelli sotto-zero tires.

You shouldn't have any problem to fit these, as only IS350 has bigger rotors on front brakes.