The D7100 does have a small buffer, but if you use a fast SD card (e.g., a 95 MB/s Sandisk Extreme) it clears pretty quickly.
I used to have the 50-500, but mainly used it with the D7000; I sold it before I went to the D7100, and have since sold the D7100 as well. It should be even better with the 7100 or 7200 and the 51-point AF system. The 50-500 is a very versatile lens with pretty good autofocus speed even at the long end. Sharpness is decent, particularly if you stop down to f/8 for optimal sharpness at the long end. Here are some old shots I took with the 50-500:
https://flic.kr/s/aHsjw8gQ2e