need advice with Bilstein B6 front setup
 

I just received Bilstein B6 front setup with H&R sport lowering springs. Given that I have rear only air suspension. I have 2 questions :

1. what to supplement the rear setup given 136K on prob Original equipment. What do most owners with B6 setup have in the rear?

2. I currently have 19" wheel staggered setup. What would be the pro's to go with 20" wheel setup from a performance base?

Thanks