The Sonic can have some pretty good pull to it, especially when it loads up and is in the stronger winds (above 8mph). I wouldn't fly it on anything less than 150#. I also don't care much for it in anything under about 7-8 mph and it really kicks when the winds hit 10. Personally, that's when this kite becomes a rush to fly. Absolutely awesome and fast.....I mean really fast!
85' lines should be perfect. This kite has such a high speed across the window that shorter lines will be a bit more difficult. Longer lines will slow it down a bit and give you much more room in the sky to fly. To each their own though....
I would use your 85' lines and go for it. The Sonic does require a little quad line skill, not one that I would recommend starting out with if you've never flown before. Also, give yourself some time to really get use to it, feather the rear lines and get it locked in and get use to how that feels. With a little time, you may find it as one of your favorite kites to fly. I did.
Hope that helps.