1) Book on 28th june at PRS/IRCTC online.
2) You will get lower berth since you are booking early.
3) I had travelled in Coro during May in 3AC from Vizag to KGP.The journey was good and comfortable.though we have to fight with attendant for new set of bedding and with TC(2 people have occupied our bearth),rest of the journey was comfortable.I would suggest to go with coro.