White looks good on black, but i reccomend the P1s in the Bronze, or even Gun metal.
As for tyre size, you can go down to 40 man, i had 40 profile on my 17s on the last car.
I run 35s on the 18s i have, they are 18x10 and 18x9 with 255/35 and 265/35 just for a reference.
You can probably stretch 235/40 onto the back, but i'd say go for something around 245/45 perhaps. But ask your tyre shop, they'll be able to say for sure what you can and cannot fit.