Hello everyone just going to ask a couple of newbie questions I'm after some after market wheels most likely stick with 18 but just after the standard rim widths and offsets so I can look out for what will fit thanks or can I got 19 with lower profile tyres and the speedo be the same ? thanks for your time

 

rear 18" for is350 is 50mm offset for 8.5" rim.
front 18" for is350 is 45mm offset for 8.0" rim.

I think you will find those offsets difficult to match in an after market rim. If you stick with 255 rear and 225 front you have some leeway on the offsets you can get away with. There is a "will those wheels fit" thread. I'd start there to figure out what to look for.

Lumarai sells rims specifically for is350, so they SHOULD fit. You can also check their options as a place to begin to see what you can use for front and rear widths and offsets.