I think you are lucky to enroll Part B before. Without it, your part B would not cover your post-transplant drugs. You have to enroll Part B before or at the time of transplant for it to cover the drugs.
If you qualify for Part A, you will also be offered Part B. Remember, they are both needed to get all the Medicare benefits available for ESRD, and there is a premium for Part B. If you decide not to get Part B right away, you can only get it between January 1–March 31 each year. Your coverage will begin on July 1. Also, the cost of Part B will go up 10% for each 12-month period that you could have had Part B but didn’t sign up for it.