I'd start with making sure the car seat for the 18mo old fits, especially if still rear-facing. Might need to step up to the midsize (ES3xx blah mobile) or GS if a Lexus. I think the G35/37 is a little bigger in the back than the IS, but not much.
I too am a fan of the G35/37 sound. I really like the G35 coupe, but that would be not so practical for car seat usage.
For 10K ish, get a used LS400/430
