Do they plan on eradicating the cause of your problems at the time of the op or is it strictly for diagnostic purposes. My advice is make sure the surgeon is familiar with endometriosis excision and hopefully prefers that method to cauterization as it supposed to be far more successful at eliminating endo.
If it is endo, chances are you will require more surgeries down the road at your age. The surgeons are more reluctant to perform surgeries repeatedly due to further complicating matters with scarring tissue being susceptable for collection sites for endo and the risk of infection and anethesia. So if I were you I wouldn't go for just a diagnosis unless they plan on eradicating it then and there.
You didn't say what diagnostic tests you were sent for prior to know what you have ruled out so far. Did you have a colonoscopy to see if there is another cause to your bleeding of the bowels, are you up to date on your pap smears, did you have abdominal ultrasound to view the kidneys.
Anyway your lucky to have a gyne who is considering endo to be the cause and is willing to do surgery not just mask the pain, it is most often a struggle to get them to that point.
I'm curious have you already gone along with there sidetracking methods to get to this point? Is you pain debilitating your ability to do your daily tasks or are you trying to conceive?
I ask because a lot of us can't get our gynecologists to cooperate by surgical means, especially on the first appointment. Had you been to see your several times?
Take it easy after the op and allow your body to heal well. Go and visit your gyne once you are well enough to do so to determine what was discovered. Have them explain it all to you cause personally I didn't understand any of it and only cared that it helped elliminate my pain. I didn't know at the time of the possibilty of it returning and there were a lot of things found that I still don't have a good understanding of. They won't cover it all in your discharge papers and you'll be too groggy to really comprehend from the anesthesia when the surgeon speaks to you after the op.