Hi! Welcome to the forums!
I also only have one kidney. I had Wilms Tumor as a child and had a right kidney nephrectomy when I was 6yrs old. I was also on labetalol while pg. Alison had no bp issues after birth, and you can breastfeed while on labetalol. My experience was that when I was pg, the labetalol made the baby less reactive during things like NST's, which often led to more testing, but in the end, she always passed those. The MFM told me that was a common side effect which makes sense since it often made me sleepy too after I would take it. I have seen no long term side effects in Alison and she is now 18months.
As for the kidney, I would suggest making sure to get a baseline 24hour urine before pg, and imediately after you find out you are pg. That way they will know exactly the level of function your kidney has before pg and can measure any changes as the pg moves along. After my first PE pg with Abby, my kidney returned to completely normal function (it had gone into failure) within six weeks and I had no issues with lower function in my second pg.
HTH and feel free to message me directly if you have any other questions! Also, I would love to know how you came to only have one kidney if you wouldn't mind sharing.