I finally decided that the IS200 will run on Gram Lights T57-RC wheels (silver), but I can't seem to figure out the size I should get.
The wheel is available in 18", 7.5J and 8.5J.
My goal is obviously to go as large as possible, with as few modifications as possible.
The car will be lowered (TTE coilovers most likely) to about half an inch between the top of the tire and the wheel arch, but the bumpers will stay stock. Similar to sparkystavs.

I just want it to look like they fill out the arches, but need to keep within +/-3% tyre diameter, not to mention the adverse effect on acceleration a larger diameter tyre will have.

I guess I just need someone to tell me what to get

7.5 front 8.5 rear and you will not need to modify anything

That sounds like a good idea to me

make sure you can get the correct offset

Sounds like a good idea to me too, until I remember that the MOT guys here require same size tires front and rear unless the car was born with different ones... (Yes, they are incredibly restrictive here)

How big of a problem is it getting 8.5's to fit in the front? offset is +45

8.5 should fit with ET45, only issue is that you will not be able to turn full lock as your tire will hit the inside arch. To be safe you have to use 225 max in front