I have a complex endophytic left renal cyst. Medial cortex just under the ureter opening. How are my chances of keeping most of my kidney?

You can keep it. even if for a solid mass they usually try to save as much kidney as possible. so for a cystic lesion, it is safer to save more kidney. so unless they are concern about malignancy they usually save the kidney.
Need only one. most can live with only one kidney for a life time. You have noted a kidney with a renal cyst. If the cyst is not compressing the ureter and the cortical structure is normal than the kidney may have normal function. You should talk to the kidney doctor who ordered the study and discuss your concerns so as to better understand. Hope this helps .