Firstly welcome from UK and grats on the car! I am in a similar position having just bought a USB IS-F although it was a MY2010, I totally agree with your decision making for going to Lexus, I've owned an MB, BMW and a Lexus RX no-ne could come close to the customer service and reliability of Lexus which made buying choice easy as it was any car from Lexus
Anyway to your questions, the ML stereo is lacking in bass however I still find it a very capable stereo and more than enough for my needs. For your music types I wuld imagine it will be perfect.
You can just keep the normal wheels on and buy winter tyres for them. Probably easier to get over there ( ordered mine when I bought the car at end Dec and was told prob won't get them before the winter has ended!
Can't help on suspension.
So far white leather has been fine but then haven't had the car long enough to properly evaluate. Did get the interior protected though just to help make sure jeans don't colour them.
Nothing like that with IS-F so shouldn't worry about it. The brakes are superb and every review has always praised them as one of the best brakes with almost no fade and great stopping power. Only thing to watch is brake dust embedding in the wheels and side of the car, need to clean regularly especially after a hard run.