Dear Scared,
I think all of us on this forum can identify with your feelings! I thought long and hard about donating to my Brother, and almost didn't. There are a couple things I wish I would have done differently, and I would like to tell you what they are:
1. Apply for and obtain life insurance long before donation.
2. Take all of your test results to a nephrologist, not associated with a transplant center, and get a 2nd opinion on your eligibility as a donor.
One thing I did not totally realize before my surgery was that... if complications arise years down the line... you have to pay for any needed medical procedures yourself. If you have good health insurance, this is not a problem, but something to consider.
My donation was a very positive thing. My brother went from spending most of his time doing dialysis (seriously, it was depressing), to pretty much perfect health at this point. He doesn't even take any anti-rejection meds anymore.
It is a great thing you can do, but you are right to also think of your own long-term consequences.
kdub