What do you get as a lvl 7 champion

You can only get 1 chest per champion per season. If you got a chest while playing that champion you have to play something else.

Playing the champion in general and getting S doesn't do anything but its a cool reminder why you're level 7 with that champion. You'll only get more mastery points.

You specifically don't have to get an s for the chest, only someone on your team including yourself, just playing normally even on a champ you're bad at will earn you a chest.

I'm not digging for the source, but I'll just post my evidence. I've gotten a chest as lux with only an A+ as my seasonal highest grade http://i.imgur.com/OfKxFjA.png I've gotten a chest as lee with only a B+ as my seasonal highest grade http://i.imgur.com/Rxb0mB7.png and for one more, my seasonal highest for my jinx was an A+ and i've unlocked a chest from her. http://i.imgur.com/AZ4ngdG.png and I'm not going to bother imaging, but for reference I got a chest as malzahar with an A+ highest grade, Morgana with an A+, and raka with an A highest grade.

S rank on that champion isn't needed for the chest, as long as a teammate gets an S- or above in your match you'll get a chest while playing that champion if you haven't before.

Hextech chest don't require you to get an S as that champion, people struggle getting an S- on their mains sometimes, let alone on other champions because their mains already unlocked a chest. It would be way too limiting if that was the case.